diff --git a/config.def.h b/config.def.h
index 6f05dce..7ae1b92 100644
--- a/config.def.h
+++ b/config.def.h
@@ -470,3 +470,27 @@ static char ascii_printable[] =
 	" !\"#$%&'()*+,-./0123456789:;<=>?"
 	"@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_"
 	"`abcdefghijklmnopqrstuvwxyz{|}~";
+
+/**
+ * Undercurl style. Set UNDERCURL_STYLE to one of the available styles.
+ *
+ * Curly: Dunno how to draw it *shrug*
+ *  _   _   _   _
+ * ( ) ( ) ( ) ( )
+ *	 (_) (_) (_) (_)
+ *
+ * Spiky:
+ * /\  /\   /\	/\
+ *   \/  \/	  \/
+ *
+ * Capped:
+ *	_     _     _
+ * / \   / \   / \
+ *    \_/   \_/
+ */
+// Available styles
+#define UNDERCURL_CURLY 0
+#define UNDERCURL_SPIKY 1
+#define UNDERCURL_CAPPED 2
+// Active style
+#define UNDERCURL_STYLE UNDERCURL_SPIKY
diff --git a/st.c b/st.c
index 76b7e0d..542ab3a 100644
--- a/st.c
+++ b/st.c
@@ -33,6 +33,7 @@
 #define UTF_SIZ       4
 #define ESC_BUF_SIZ   (128*UTF_SIZ)
 #define ESC_ARG_SIZ   16
+#define CAR_PER_ARG   4
 #define STR_BUF_SIZ   ESC_BUF_SIZ
 #define STR_ARG_SIZ   ESC_ARG_SIZ
 
@@ -139,6 +140,7 @@ typedef struct {
 	int arg[ESC_ARG_SIZ];
 	int narg;              /* nb of args */
 	char mode[2];
+	int carg[ESC_ARG_SIZ][CAR_PER_ARG]; /* colon args */
 } CSIEscape;
 
 /* STR Escape sequence structs */
@@ -159,6 +161,7 @@ static void ttywriteraw(const char *, size_t);
 
 static void csidump(void);
 static void csihandle(void);
+static void readcolonargs(char **, int, int[][CAR_PER_ARG]);
 static void csiparse(void);
 static void csireset(void);
 static int eschandle(uchar);
@@ -1131,6 +1134,28 @@ tnewline(int first_col)
 	tmoveto(first_col ? 0 : term.c.x, y);
 }
 
+void
+readcolonargs(char **p, int cursor, int params[][CAR_PER_ARG])
+{
+	int i = 0;
+	for (; i < CAR_PER_ARG; i++)
+		params[cursor][i] = -1;
+
+	if (**p != ':')
+		return;
+
+	char *np = NULL;
+	i = 0;
+
+	while (**p == ':' && i < CAR_PER_ARG) {
+		while (**p == ':')
+			(*p)++;
+		params[cursor][i] = strtol(*p, &np, 10);
+		*p = np;
+		i++;
+	}
+}
+
 void
 csiparse(void)
 {
@@ -1153,6 +1178,7 @@ csiparse(void)
 			v = -1;
 		csiescseq.arg[csiescseq.narg++] = v;
 		p = np;
+		readcolonargs(&p, csiescseq.narg-1, csiescseq.carg);
 		if (*p != ';' || csiescseq.narg == ESC_ARG_SIZ)
 			break;
 		p++;
@@ -1369,6 +1395,10 @@ tsetattr(int *attr, int l)
 				ATTR_STRUCK     );
 			term.c.attr.fg = defaultfg;
 			term.c.attr.bg = defaultbg;
+			term.c.attr.ustyle = -1;
+			term.c.attr.ucolor[0] = -1;
+			term.c.attr.ucolor[1] = -1;
+			term.c.attr.ucolor[2] = -1;
 			break;
 		case 1:
 			term.c.attr.mode |= ATTR_BOLD;
@@ -1380,7 +1410,14 @@ tsetattr(int *attr, int l)
 			term.c.attr.mode |= ATTR_ITALIC;
 			break;
 		case 4:
-			term.c.attr.mode |= ATTR_UNDERLINE;
+			term.c.attr.ustyle = csiescseq.carg[i][0];
+
+			if (term.c.attr.ustyle != 0)
+				term.c.attr.mode |= ATTR_UNDERLINE;
+			else
+				term.c.attr.mode &= ~ATTR_UNDERLINE;
+
+			term.c.attr.mode ^= ATTR_DIRTYUNDERLINE;
 			break;
 		case 5: /* slow blink */
 			/* FALLTHROUGH */
@@ -1431,6 +1468,18 @@ tsetattr(int *attr, int l)
 		case 49:
 			term.c.attr.bg = defaultbg;
 			break;
+		case 58:
+			term.c.attr.ucolor[0] = csiescseq.carg[i][1];
+			term.c.attr.ucolor[1] = csiescseq.carg[i][2];
+			term.c.attr.ucolor[2] = csiescseq.carg[i][3];
+			term.c.attr.mode ^= ATTR_DIRTYUNDERLINE;
+			break;
+		case 59:
+			term.c.attr.ucolor[0] = -1;
+			term.c.attr.ucolor[1] = -1;
+			term.c.attr.ucolor[2] = -1;
+			term.c.attr.mode ^= ATTR_DIRTYUNDERLINE;
+			break;
 		default:
 			if (BETWEEN(attr[i], 30, 37)) {
 				term.c.attr.fg = attr[i] - 30;
diff --git a/st.h b/st.h
index 3d351b6..95bdcbd 100644
--- a/st.h
+++ b/st.h
@@ -34,6 +34,7 @@ enum glyph_attribute {
 	ATTR_WIDE       = 1 << 9,
 	ATTR_WDUMMY     = 1 << 10,
 	ATTR_BOLD_FAINT = ATTR_BOLD | ATTR_FAINT,
+	ATTR_DIRTYUNDERLINE = 1 << 15,
 };
 
 enum selection_mode {
@@ -65,6 +66,8 @@ typedef struct {
 	ushort mode;      /* attribute flags */
 	uint32_t fg;      /* foreground  */
 	uint32_t bg;      /* background  */
+	int ustyle;	  /* underline style */
+	int ucolor[3];    /* underline color */
 } Glyph;
 
 typedef Glyph *Line;
diff --git a/st.info b/st.info
index 8201ad6..659878c 100644
--- a/st.info
+++ b/st.info
@@ -1,4 +1,5 @@
 st-mono| simpleterm monocolor,
+	Su,
 	acsc=+C\,D-A.B0E``aaffgghFiGjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,
 	am,
 	bce,
diff --git a/x.c b/x.c
index 210f184..3a0e79e 100644
--- a/x.c
+++ b/x.c
@@ -45,6 +45,14 @@ typedef struct {
 	signed char appcursor; /* application cursor */
 } Key;
 
+/* Undercurl slope types */
+enum undercurl_slope_type {
+	UNDERCURL_SLOPE_ASCENDING = 0,
+	UNDERCURL_SLOPE_TOP_CAP = 1,
+	UNDERCURL_SLOPE_DESCENDING = 2,
+	UNDERCURL_SLOPE_BOTTOM_CAP = 3
+};
+
 /* X modifiers */
 #define XK_ANY_MOD    UINT_MAX
 #define XK_NO_MOD     0
@@ -1339,6 +1347,51 @@ xmakeglyphfontspecs(XftGlyphFontSpec *specs, const Glyph *glyphs, int len, int x
 	return numspecs;
 }
 
+static int isSlopeRising (int x, int iPoint, int waveWidth)
+{
+	//    .     .     .     .
+	//   / \   / \   / \   / \
+	//  /   \ /   \ /   \ /   \
+	// .     .     .     .     .
+
+	// Find absolute `x` of point
+	x += iPoint * (waveWidth/2);
+
+	// Find index of absolute wave
+	int absSlope = x / ((float)waveWidth/2);
+
+	return (absSlope % 2);
+}
+
+static int getSlope (int x, int iPoint, int waveWidth)
+{
+	// Sizes: Caps are half width of slopes
+	//    1_2       1_2       1_2      1_2
+	//   /   \     /   \     /   \    /   \
+	//  /     \   /     \   /     \  /     \
+	// 0       3_0       3_0      3_0       3_
+	// <2->    <1>         <---6---->
+
+	// Find type of first point
+	int firstType;
+	x -= (x / waveWidth) * waveWidth;
+	if (x < (waveWidth * (2.f/6.f)))
+		firstType = UNDERCURL_SLOPE_ASCENDING;
+	else if (x < (waveWidth * (3.f/6.f)))
+		firstType = UNDERCURL_SLOPE_TOP_CAP;
+	else if (x < (waveWidth * (5.f/6.f)))
+		firstType = UNDERCURL_SLOPE_DESCENDING;
+	else
+		firstType = UNDERCURL_SLOPE_BOTTOM_CAP;
+
+	// Find type of given point
+	int pointType = (iPoint % 4);
+	pointType += firstType;
+	pointType %= 4;
+
+	return pointType;
+}
+
 void
 xdrawglyphfontspecs(const XftGlyphFontSpec *specs, Glyph base, int len, int x, int y)
 {
@@ -1461,8 +1514,357 @@ xdrawglyphfontspecs(const XftGlyphFontSpec *specs, Glyph base, int len, int x, i
 
 	/* Render underline and strikethrough. */
 	if (base.mode & ATTR_UNDERLINE) {
-		XftDrawRect(xw.draw, fg, winx, winy + dc.font.ascent + 1,
-				width, 1);
+		// Underline Color
+		const int widthThreshold  = 28; // +1 width every widthThreshold px of font
+		int wlw = (win.ch / widthThreshold) + 1; // Wave Line Width
+		int linecolor;
+		if ((base.ucolor[0] >= 0) &&
+			!(base.mode & ATTR_BLINK && win.mode & MODE_BLINK) &&
+			!(base.mode & ATTR_INVISIBLE)
+		) {
+			// Special color for underline
+			// Index
+			if (base.ucolor[1] < 0) {
+				linecolor = dc.col[base.ucolor[0]].pixel;
+			}
+			// RGB
+			else {
+				XColor lcolor;
+				lcolor.red = base.ucolor[0] * 257;
+				lcolor.green = base.ucolor[1] * 257;
+				lcolor.blue = base.ucolor[2] * 257;
+				lcolor.flags = DoRed | DoGreen | DoBlue;
+				XAllocColor(xw.dpy, xw.cmap, &lcolor);
+				linecolor = lcolor.pixel;
+			}
+		} else {
+			// Foreground color for underline
+			linecolor = fg->pixel;
+		}
+
+		XGCValues ugcv = {
+			.foreground = linecolor,
+			.line_width = wlw,
+			.line_style = LineSolid,
+			.cap_style = CapNotLast
+		};
+
+		GC ugc = XCreateGC(xw.dpy, XftDrawDrawable(xw.draw),
+			GCForeground | GCLineWidth | GCLineStyle | GCCapStyle,
+			&ugcv);
+
+		// Underline Style
+		if (base.ustyle != 3) {
+			//XftDrawRect(xw.draw, fg, winx, winy + dc.font.ascent + 1, width, 1);
+			XFillRectangle(xw.dpy, XftDrawDrawable(xw.draw), ugc, winx,
+				winy + dc.font.ascent + 1, width, wlw);
+		} else if (base.ustyle == 3) {
+			int ww = win.cw;//width;
+			int wh = dc.font.descent - wlw/2 - 1;//r.height/7;
+			int wx = winx;
+			int wy = winy + win.ch - dc.font.descent;
+
+#if UNDERCURL_STYLE == UNDERCURL_CURLY
+			// Draw waves
+			int narcs = charlen * 2 + 1;
+			XArc *arcs = xmalloc(sizeof(XArc) * narcs);
+
+			int i = 0;
+			for (i = 0; i < charlen-1; i++) {
+				arcs[i*2] = (XArc) {
+					.x = wx + win.cw * i + ww / 4,
+					.y = wy,
+					.width = win.cw / 2,
+					.height = wh,
+					.angle1 = 0,
+					.angle2 = 180 * 64
+				};
+				arcs[i*2+1] = (XArc) {
+					.x = wx + win.cw * i + ww * 0.75,
+					.y = wy,
+					.width = win.cw/2,
+					.height = wh,
+					.angle1 = 180 * 64,
+					.angle2 = 180 * 64
+				};
+			}
+			// Last wave
+			arcs[i*2] = (XArc) {wx + ww * i + ww / 4, wy, ww / 2, wh,
+			0, 180 * 64 };
+			// Last wave tail
+			arcs[i*2+1] = (XArc) {wx + ww * i + ww * 0.75, wy, ceil(ww / 2.),
+			wh, 180 * 64, 90 * 64};
+			// First wave tail
+			i++;
+			arcs[i*2] = (XArc) {wx - ww/4 - 1, wy, ceil(ww / 2.), wh, 270 * 64,
+			90 * 64 };
+
+			XDrawArcs(xw.dpy, XftDrawDrawable(xw.draw), ugc, arcs, narcs);
+
+			free(arcs);
+#elif UNDERCURL_STYLE == UNDERCURL_SPIKY
+			// Make the underline corridor larger
+			/*
+			wy -= wh;
+			*/
+			wh *= 2;
+
+			// Set the angle of the slope to 45°
+			ww = wh;
+
+			// Position of wave is independent of word, it's absolute
+			wx = (wx / (ww/2)) * (ww/2);
+
+			int marginStart = winx - wx;
+
+			// Calculate number of points with floating precision
+			float n = width;					// Width of word in pixels
+			n = (n / ww) * 2;					// Number of slopes (/ or \)
+			n += 2;								// Add two last points
+			int npoints = n;					// Convert to int
+
+			// Total length of underline
+			float waveLength = 0;
+
+			if (npoints >= 3) {
+				// We add an aditional slot in case we use a bonus point
+				XPoint *points = xmalloc(sizeof(XPoint) * (npoints + 1));
+
+				// First point (Starts with the word bounds)
+				points[0] = (XPoint) {
+					.x = wx + marginStart,
+					.y = (isSlopeRising(wx, 0, ww))
+						? (wy - marginStart + ww/2.f)
+						: (wy + marginStart)
+				};
+
+				// Second point (Goes back to the absolute point coordinates)
+				points[1] = (XPoint) {
+					.x = (ww/2.f) - marginStart,
+					.y = (isSlopeRising(wx, 1, ww))
+						? (ww/2.f - marginStart)
+						: (-ww/2.f + marginStart)
+				};
+				waveLength += (ww/2.f) - marginStart;
+
+				// The rest of the points
+				for (int i = 2; i < npoints-1; i++) {
+					points[i] = (XPoint) {
+						.x = ww/2,
+						.y = (isSlopeRising(wx, i, ww))
+							? wh/2
+							: -wh/2
+					};
+					waveLength += ww/2;
+				}
+
+				// Last point
+				points[npoints-1] = (XPoint) {
+					.x = ww/2,
+					.y = (isSlopeRising(wx, npoints-1, ww))
+						? wh/2
+						: -wh/2
+				};
+				waveLength += ww/2;
+
+				// End
+				if (waveLength < width) { // Add a bonus point?
+					int marginEnd = width - waveLength;
+					points[npoints] = (XPoint) {
+						.x = marginEnd,
+						.y = (isSlopeRising(wx, npoints, ww))
+							? (marginEnd)
+							: (-marginEnd)
+					};
+
+					npoints++;
+				} else if (waveLength > width) { // Is last point too far?
+					int marginEnd = waveLength - width;
+					points[npoints-1].x -= marginEnd;
+					if (isSlopeRising(wx, npoints-1, ww))
+						points[npoints-1].y -= (marginEnd);
+					else
+						points[npoints-1].y += (marginEnd);
+				}
+
+				// Draw the lines
+				XDrawLines(xw.dpy, XftDrawDrawable(xw.draw), ugc, points, npoints,
+						CoordModePrevious);
+
+				// Draw a second underline with an offset of 1 pixel
+				if ( ((win.ch / (widthThreshold/2)) % 2)) {
+					points[0].x++;
+
+					XDrawLines(xw.dpy, XftDrawDrawable(xw.draw), ugc, points,
+							npoints, CoordModePrevious);
+				}
+
+				// Free resources
+				free(points);
+			}
+#else // UNDERCURL_CAPPED
+			// Cap is half of wave width
+			float capRatio = 0.5f;
+
+			// Make the underline corridor larger
+			wh *= 2;
+
+			// Set the angle of the slope to 45°
+			ww = wh;
+			ww *= 1 + capRatio; // Add a bit of width for the cap
+
+			// Position of wave is independent of word, it's absolute
+			wx = (wx / ww) * ww;
+
+			float marginStart;
+			switch(getSlope(winx, 0, ww)) {
+				case UNDERCURL_SLOPE_ASCENDING:
+					marginStart = winx - wx;
+					break;
+				case UNDERCURL_SLOPE_TOP_CAP:
+					marginStart = winx - (wx + (ww * (2.f/6.f)));
+					break;
+				case UNDERCURL_SLOPE_DESCENDING:
+					marginStart = winx - (wx + (ww * (3.f/6.f)));
+					break;
+				case UNDERCURL_SLOPE_BOTTOM_CAP:
+					marginStart = winx - (wx + (ww * (5.f/6.f)));
+					break;
+			}
+
+			// Calculate number of points with floating precision
+			float n = width;					// Width of word in pixels
+												//					   ._.
+			n = (n / ww) * 4;					// Number of points (./   \.)
+			n += 2;								// Add two last points
+			int npoints = n;					// Convert to int
+
+			// Position of the pen to draw the lines
+			float penX = 0;
+			float penY = 0;
+
+			if (npoints >= 3) {
+				XPoint *points = xmalloc(sizeof(XPoint) * (npoints + 1));
+
+				// First point (Starts with the word bounds)
+				penX = winx;
+				switch (getSlope(winx, 0, ww)) {
+					case UNDERCURL_SLOPE_ASCENDING:
+						penY = wy + wh/2.f - marginStart;
+						break;
+					case UNDERCURL_SLOPE_TOP_CAP:
+						penY = wy;
+						break;
+					case UNDERCURL_SLOPE_DESCENDING:
+						penY = wy + marginStart;
+						break;
+					case UNDERCURL_SLOPE_BOTTOM_CAP:
+						penY = wy + wh/2.f;
+						break;
+				}
+				points[0].x = penX;
+				points[0].y = penY;
+
+				// Second point (Goes back to the absolute point coordinates)
+				switch (getSlope(winx, 1, ww)) {
+					case UNDERCURL_SLOPE_ASCENDING:
+						penX += ww * (1.f/6.f) - marginStart;
+						penY += 0;
+						break;
+					case UNDERCURL_SLOPE_TOP_CAP:
+						penX += ww * (2.f/6.f) - marginStart;
+						penY += -wh/2.f + marginStart;
+						break;
+					case UNDERCURL_SLOPE_DESCENDING:
+						penX += ww * (1.f/6.f) - marginStart;
+						penY += 0;
+						break;
+					case UNDERCURL_SLOPE_BOTTOM_CAP:
+						penX += ww * (2.f/6.f) - marginStart;
+						penY += -marginStart + wh/2.f;
+						break;
+				}
+				points[1].x = penX;
+				points[1].y = penY;
+
+				// The rest of the points
+				for (int i = 2; i < npoints; i++) {
+					switch (getSlope(winx, i, ww)) {
+						case UNDERCURL_SLOPE_ASCENDING:
+						case UNDERCURL_SLOPE_DESCENDING:
+							penX += ww * (1.f/6.f);
+							penY += 0;
+							break;
+						case UNDERCURL_SLOPE_TOP_CAP:
+							penX += ww * (2.f/6.f);
+							penY += -wh / 2.f;
+							break;
+						case UNDERCURL_SLOPE_BOTTOM_CAP:
+							penX += ww * (2.f/6.f);
+							penY += wh / 2.f;
+							break;
+					}
+					points[i].x = penX;
+					points[i].y = penY;
+				}
+
+				// End
+				float waveLength = penX - winx;
+				if (waveLength < width) { // Add a bonus point?
+					int marginEnd = width - waveLength;
+					penX += marginEnd;
+					switch(getSlope(winx, npoints, ww)) {
+						case UNDERCURL_SLOPE_ASCENDING:
+						case UNDERCURL_SLOPE_DESCENDING:
+							//penY += 0;
+							break;
+						case UNDERCURL_SLOPE_TOP_CAP:
+							penY += -marginEnd;
+							break;
+						case UNDERCURL_SLOPE_BOTTOM_CAP:
+							penY += marginEnd;
+							break;
+					}
+
+					points[npoints].x = penX;
+					points[npoints].y = penY;
+
+					npoints++;
+				} else if (waveLength > width) { // Is last point too far?
+					int marginEnd = waveLength - width;
+					points[npoints-1].x -= marginEnd;
+					switch(getSlope(winx, npoints-1, ww)) {
+						case UNDERCURL_SLOPE_TOP_CAP:
+							points[npoints-1].y += marginEnd;
+							break;
+						case UNDERCURL_SLOPE_BOTTOM_CAP:
+							points[npoints-1].y -= marginEnd;
+							break;
+						default:
+							break;
+					}
+				}
+
+				// Draw the lines
+				XDrawLines(xw.dpy, XftDrawDrawable(xw.draw), ugc, points, npoints,
+						CoordModeOrigin);
+
+				// Draw a second underline with an offset of 1 pixel
+				if ( ((win.ch / (widthThreshold/2)) % 2)) {
+					for (int i = 0; i < npoints; i++)
+						points[i].x++;
+
+					XDrawLines(xw.dpy, XftDrawDrawable(xw.draw), ugc, points,
+							npoints, CoordModeOrigin);
+				}
+
+				// Free resources
+				free(points);
+			}
+#endif
+		}
+
+		XFreeGC(xw.dpy, ugc);
 	}
 
 	if (base.mode & ATTR_STRUCK) {