diff --git a/src/Editor.css b/src/Editor.css
index c0e6015..3a2f1a6 100644
--- a/src/Editor.css
+++ b/src/Editor.css
@@ -49,4 +49,9 @@
   height: 100%;
   pointer-events: none;
   color: transparent;
+  white-space: pre-wrap;
+
+  .highlighted {
+    background: #ffff00;
+  }
 }
diff --git a/src/Editor.tsx b/src/Editor.tsx
index f7cc5ee..b61c695 100644
--- a/src/Editor.tsx
+++ b/src/Editor.tsx
@@ -1,6 +1,7 @@
-import React, { useState } from "react";
+import React, { ReactNode, useState } from "react";
 import "./Editor.css";
 import { Highlight } from "./highlight";
+import { buildShadow } from "./shadow";
 
 function Editor({ defaultValue = "I have a text value." }) {
   function handleChange(event: React.ChangeEvent<HTMLTextAreaElement>) {
@@ -13,17 +14,19 @@ function Editor({ defaultValue = "I have a text value." }) {
   const [highlights, setHighlights] = useState<Array<Highlight>>([]);
 
   function addHighlight(start: number, end: number) {
-    setHighlights([...highlights, new Highlight(start, end)]);
+    let new_highlights = [...highlights, new Highlight(start, end)];
+    new_highlights.sort(function (a,b) {
+      if (a.start < b.start) return -1;
+      if (a.start > b.start) return 1;
+      return 0;
+    });
+    setHighlights(new_highlights);
   }
 
   function clearHighlights() {
     setHighlights([]);
   }
 
-  function buildShadow() {
-    // foo
-  }
-
   if (highlights.length === 0) {
     addHighlight(1, 5);
   }
@@ -36,7 +39,7 @@ function Editor({ defaultValue = "I have a text value." }) {
           className="Editor-textarea"
           value={value}
         />
-        <div className="Editor-underlay">{value}</div>
+        <div className="Editor-underlay">{buildShadow(highlights, value)}</div>
       </div>
       <div className="Editor-ast"></div>
     </div>
diff --git a/src/shadow.tsx b/src/shadow.tsx
new file mode 100644
index 0000000..acc1e3d
--- /dev/null
+++ b/src/shadow.tsx
@@ -0,0 +1,69 @@
+import React, { ReactNode, useState } from "react";
+import { Highlight } from "./highlight";
+
+function buildShadow(highlights: Highlight[], text: string): ReactNode[] {
+  let remaining_highlights = highlights.slice();
+  let output: ReactNode[] = [];
+  let i = 0;
+  let state = ShadowState.Text;
+  let buffer = "";
+  for (let chr of text) {
+    if (state == ShadowState.Text) {
+      if (remaining_highlights.length > 0 && i == remaining_highlights[0].start) {
+        // Start a span
+        output.push(buffer);
+        buffer = chr;
+        state = ShadowState.Highlight;
+      } else {
+        // Add a character
+        buffer += chr;
+      }
+    } else if (state == ShadowState.Highlight) {
+      if (remaining_highlights.length > 0 && i == remaining_highlights[0].end) {
+        // End the span
+        output.push(<span className="highlighted">{buffer}</span>);
+        buffer = chr;
+        state = ShadowState.Text;
+        remaining_highlights = remaining_highlights.slice(1);
+      } else {
+        // Add a character
+        buffer += chr;
+      }
+    }
+    ++i;
+  }
+
+  if (buffer.length > 0) {
+    if (state == ShadowState.Text) {
+        output.push(buffer);
+    } else if (state == ShadowState.Highlight) {
+        output.push(<span className="highlighted">{buffer}</span>);
+    }
+  }
+  return output;
+}
+
+function unicodeAwareSlice(text: string, start: number, end: number): string {
+  // Boooo javascript
+  let i = 0;
+  let output = "";
+  for (let chr of text) {
+    if (i >= end) {
+      break;
+    }
+    if (i >= start) {
+      output += chr;
+    }
+    ++i;
+  }
+  return output;
+}
+
+const enum ShadowState {
+  Text,
+  Highlight,
+}
+
+export {
+buildShadow
+}