diff --git a/scripts/utils/prosediff.php b/scripts/utils/prosediff.php
new file mode 100755
--- /dev/null
+++ b/scripts/utils/prosediff.php
@@ -0,0 +1,50 @@
+#!/usr/bin/env php
+<?php
+
+require_once dirname(__FILE__).'/../__init_script__.php';
+
+$args = new PhutilArgumentParser($argv);
+$args->setTagline(pht('show prose differences'));
+$args->setSynopsis(<<<EOHELP
+**prosediff.php** __old__ __new__ [__options__]
+    Diff two prose files.
+
+EOHELP
+);
+
+$args->parseStandardArguments();
+$args->parse(
+  array(
+    array(
+      'name'      => 'files',
+      'wildcard'  => true,
+    ),
+  ));
+
+$files = $args->getArg('files');
+if (count($files) !== 2) {
+  $args->printHelpAndExit();
+}
+$old_file = head($files);
+$new_file = last($files);
+
+$old_data = Filesystem::readFile($old_file);
+$new_data = Filesystem::readFile($new_file);
+
+$engine = new PhutilProseDifferenceEngine();
+
+$prose_diff = $engine->getDiff($old_data, $new_data);
+
+foreach ($prose_diff->getParts() as $part) {
+  switch ($part['type']) {
+    case '-':
+      echo tsprintf('<bg:red>%B</bg>', $part['text']);
+      break;
+    case '+':
+      echo tsprintf('<bg:green>%B</bg>', $part['text']);
+      break;
+    case '=':
+      echo tsprintf('%B', $part['text']);
+      break;
+  }
+}
diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -298,6 +298,8 @@
     'PhutilPortuguesePortugalLocale' => 'internationalization/locales/PhutilPortuguesePortugalLocale.php',
     'PhutilPregsprintfTestCase' => 'xsprintf/__tests__/PhutilPregsprintfTestCase.php',
     'PhutilProcessGroupDaemon' => 'daemon/torture/PhutilProcessGroupDaemon.php',
+    'PhutilProseDiff' => 'utils/PhutilProseDiff.php',
+    'PhutilProseDifferenceEngine' => 'utils/PhutilProseDifferenceEngine.php',
     'PhutilProtocolChannel' => 'channel/PhutilProtocolChannel.php',
     'PhutilProxyException' => 'error/PhutilProxyException.php',
     'PhutilProxyIterator' => 'utils/PhutilProxyIterator.php',
@@ -859,6 +861,8 @@
     'PhutilPortuguesePortugalLocale' => 'PhutilLocale',
     'PhutilPregsprintfTestCase' => 'PhutilTestCase',
     'PhutilProcessGroupDaemon' => 'PhutilTortureTestDaemon',
+    'PhutilProseDiff' => 'Phobject',
+    'PhutilProseDifferenceEngine' => 'Phobject',
     'PhutilProtocolChannel' => 'PhutilChannelChannel',
     'PhutilProxyException' => 'Exception',
     'PhutilProxyIterator' => array(
diff --git a/src/utils/PhutilProseDiff.php b/src/utils/PhutilProseDiff.php
new file mode 100644
--- /dev/null
+++ b/src/utils/PhutilProseDiff.php
@@ -0,0 +1,89 @@
+<?php
+
+final class PhutilProseDiff extends Phobject {
+
+  private $parts = array();
+
+  public function addPart($type, $text) {
+    $this->parts[] = array(
+      'type' => $type,
+      'text' => $text,
+    );
+    return $this;
+  }
+
+  public function getParts() {
+    return $this->parts;
+  }
+
+  public function reorderParts() {
+    // Reorder sequences of removed and added sections to put all the "-"
+    // parts together first, then all the "+" parts together. This produces
+    // a more human-readable result than intermingling them.
+    $o_run = array();
+    $n_run = array();
+    $result = array();
+    foreach ($this->parts as $part) {
+      $type = $part['type'];
+      switch ($type) {
+        case '-':
+          $o_run[] = $part;
+          break;
+        case '+':
+          $n_run[] = $part;
+          break;
+        default:
+          foreach ($o_run as $o) {
+            $result[] = $o;
+          }
+          foreach ($n_run as $n) {
+            $result[] = $n;
+          }
+          $result[] = $part;
+          $o_run = array();
+          $n_run = array();
+          break;
+      }
+    }
+
+    foreach ($o_run as $o) {
+      $result[] = $o;
+    }
+    foreach ($n_run as $n) {
+      $result[] = $n;
+    }
+
+    // Now, combine consecuitive runs of the same type of change (like a
+    // series of "-" parts) into a single run.
+    $combined = array();
+
+    $last = null;
+    $last_text = null;
+    foreach ($result as $part) {
+      $type = $part['type'];
+
+      if ($last !== $type) {
+        $combined[] = array(
+          'type' => $last,
+          'text' => $last_text,
+        );
+        $last_text = null;
+        $last = $type;
+      }
+
+      $last_text .= $part['text'];
+    }
+
+    if ($last_text !== null) {
+      $combined[] = array(
+        'type' => $last,
+        'text' => $last_text,
+      );
+    }
+
+    $this->parts = $combined;
+
+    return $this;
+  }
+
+}
diff --git a/src/utils/PhutilProseDifferenceEngine.php b/src/utils/PhutilProseDifferenceEngine.php
new file mode 100644
--- /dev/null
+++ b/src/utils/PhutilProseDifferenceEngine.php
@@ -0,0 +1,92 @@
+<?php
+
+final class PhutilProseDifferenceEngine extends Phobject {
+
+  public function getDiff($u, $v) {
+    $diff = id(new PhutilProseDiff());
+
+    $this->buildDiff($diff, $u, $v, 1);
+    $diff->reorderParts();
+
+    return $diff;
+  }
+
+  private function buildDiff(PhutilProseDiff $diff, $u, $v, $level) {
+    if ($level == 4) {
+      $diff->addPart('-', $u);
+      $diff->addPart('+', $v);
+      return;
+    }
+
+    $u_parts = $this->splitCorpus($u, $level);
+    $v_parts = $this->splitCorpus($v, $level);
+
+    $matrix = id(new PhutilEditDistanceMatrix())
+      ->setSequences($u_parts, $v_parts)
+      ->setComputeString(true);
+
+    $u_pos = 0;
+    $v_pos = 0;
+
+    $edits = $matrix->getEditString();
+    $edits_length = strlen($edits);
+
+    for ($ii = 0; $ii < $edits_length; $ii++) {
+      $c = $edits[$ii];
+      if ($c == 's') {
+        $diff->addPart('=', $u_parts[$u_pos]);
+        $u_pos++;
+        $v_pos++;
+      } else if ($c == 'd') {
+        $diff->addPart('-', $u_parts[$u_pos]);
+        $u_pos++;
+      } else if ($c == 'i') {
+        $diff->addPart('+', $v_parts[$v_pos]);
+        $v_pos++;
+      } else if ($c == 'x') {
+        $this->buildDiff($diff, $u_parts[$u_pos], $v_parts[$v_pos], $level + 1);
+        $u_pos++;
+        $v_pos++;
+      } else {
+        throw new Exception(
+          pht(
+            'Unexpected character ("%s") in edit string.',
+            $c));
+      }
+    }
+  }
+
+  private function splitCorpus($corpus, $level) {
+    switch ($level) {
+      case 1:
+        // Level 1: Split into sentences.
+        $expr = '/([\n,!;?\.]+)/';
+        break;
+      case 2:
+        // Level 2: Split into words.
+        $expr = '/(\s+)/';
+        break;
+      case 3:
+        // Level 3: Split into characters.
+        return phutil_utf8v_combined($corpus);
+    }
+
+    $pieces = preg_split($expr, $corpus, -1, PREG_SPLIT_DELIM_CAPTURE);
+    return $this->stitchPieces($pieces);
+  }
+
+  private function stitchPieces(array $pieces) {
+    $results = array();
+    $count = count($pieces);
+    for ($ii = 0; $ii < $count; $ii += 2) {
+      $result = $pieces[$ii];
+      if ($ii + 1 < $count) {
+        $result .= $pieces[$ii + 1];
+      }
+      $results[] = $result;
+    }
+
+    return $results;
+  }
+
+}