diff --git a/trends.js b/trends.js
new file mode 100644
index 000000000..bd8c4b51d
--- /dev/null
+++ b/trends.js
@@ -0,0 +1,191 @@
+'use strict';
+
+const fs = require('fs');
+const flags = require('flags');
+const Git = require('nodegit');
+const lib = require('./lib');
+const moment = require('moment');
+
+flags.defineString('from', '2018-07-01', 'Starting date (inclusive)');
+flags.defineString('to', moment().format('YYYY-MM-DD'),
+    'Ending date (exclusive)');
+flags.defineStringList('products', ['chrome', 'firefox', 'safari'],
+    'Browsers to compare. Must match the products used on wpt.fyi');
+flags.defineString('scoring', 'binary',
+    'Method for scoring a test. Possible values are ' +
+    'binary, interop, and interop-strict. Read the source for definition.');
+flags.defineString('output', 'wpt-trends.csv',
+    'Output CSV file to write to.');
+flags.defineBoolean('experimental', false,
+    'Calculate metrics for experimental runs.');
+flags.parse();
+
+function getScoreFunc(method) {
+  switch (method) {
+    case 'binary':
+      return binaryScore;
+    case 'interop':
+      return interopScore;
+    case 'interop-strict':
+      return interopStrictScore;
+  }
+  throw new Error(`Unknown scoring method: ${method}`);
+}
+
+// Score as 0 or 1, requiring an OK harness status and all subtests passing.
+function binaryScore(status, subtests) {
+  if (status !== 'OK') {
+    return 0;
+  }
+  for (const subtest of subtests) {
+    if (subtest.status !== 'PASS') {
+      return 0;
+    }
+  }
+  return 1;
+}
+
+// Score like in Interop 2023.
+function interopScore(status, subtests) {
+  // Note: status is ignored.
+  let passingSubtests = 0;
+  for (const subtest of subtests) {
+    if (subtest.status === 'PASS') {
+      passingSubtests++;
+    }
+  }
+  return passingSubtests / subtests.length;
+}
+
+// Score like in Interop 2023, but require OK harness status.
+function interopStrictScore(status, subtests) {
+  if (status !== 'OK') {
+    return 0;
+  }
+  return interopScore(status, subtests);
+}
+
+function scoreTree(tree, scoreFunc) {
+  let score = 0;
+  let total = 0;
+
+  lib.results.walkTests(tree, (path, test, results) => {
+    if (Array.isArray(results.subtests)) {
+      // There are different ways of scoring subtests.
+      score += scoreFunc(results.status, results.subtests);
+    } else if (results.status === 'PASS') {
+      score++;
+    }
+    total++;
+  });
+
+  return [score, total];
+}
+
+async function main() {
+  // Get the scoring function to use.
+  const scoreFunc = getScoreFunc(flags.get('scoring'));
+
+  // Sort the products so that output files are consistent.
+  const products = flags.get('products');
+  if (products.length < 2) {
+    throw new Error('At least 2 products must be specified for this analysis');
+  }
+  products.sort();
+
+  const repo = await Git.Repository.open('results-analysis-cache.git');
+
+  // First, grab aligned runs from the server for the dates that we are
+  // interested in.
+  const from = moment(flags.get('from'));
+  const to = moment(flags.get('to'));
+  const experimental = flags.get('experimental');
+  const alignedRuns = await lib.runs.fetchAlignedRunsFromServer(
+      products, from, to, experimental);
+
+  // Verify that we have data for the fetched runs in the results-analysis-cache
+  // repo.
+  console.log('Getting local set of run ids from repo');
+  let before = Date.now();
+  const localRunIds = await lib.results.getLocalRunIds(repo);
+  let after = Date.now();
+  console.log(`Found ${localRunIds.size} ids (took ${after - before} ms)`);
+
+  let hadErrors = false;
+  for (const [date, runs] of alignedRuns.entries()) {
+    for (const run of runs) {
+      if (!localRunIds.has(run.id)) {
+        // If you see this, you probably need to run git-write.js or just update
+        // your results-analysis-cache.git repo; see the README.md.
+        console.error(`Run ${run.id} missing from local git repo (${date})`);
+        hadErrors = true;
+      }
+    }
+  }
+  if (hadErrors) {
+    throw new Error('Missing data for some runs (see errors logged above). ' +
+        'Try running "git fetch --all --tags" in results-analysis-cache/');
+  }
+
+  // Load the test result trees into memory; creates a list of recursive tree
+  // structures: tree = { trees: [...], tests: [...] }. Each 'tree' represents a
+  // directory, each 'test' is the results from a given test file.
+  console.log('Iterating over all runs, loading test results');
+  before = Date.now();
+  for (const runs of alignedRuns.values()) {
+    for (const run of runs) {
+      // Just in case someone ever adds a 'tree' field to the JSON.
+      if (run.tree) {
+        throw new Error('Run JSON contains "tree" field; code needs changed.');
+      }
+      run.tree = await lib.results.getGitTree(repo, run);
+    }
+  }
+  after = Date.now();
+  console.log(`Loading ${alignedRuns.size} sets of runs took ` +
+      `${after - before} ms`);
+
+  // We're ready to score the runs now!
+  console.log('Calculating scores for the runs');
+  before = Date.now();
+  const dateToScores = new Map();
+  for (const [date, runs] of alignedRuns.entries()) {
+    // The SHA should be the same for all runs, so just grab the first.
+    const sha = runs[0].full_revision_hash;
+    const versions = runs.map(run => run.browser_version);
+    const scoresAndTotals = runs.map(run => scoreTree(run.tree, scoreFunc));
+    const scores = scoresAndTotals.map(st => st[0]);
+    // TODO: getting this information from the manifest would be better, as it
+    // wouldn't depend on the results at all.
+    const tests = Math.max(...scoresAndTotals.map(st => st[1]));
+    dateToScores.set(date, {sha, versions, scores, tests});
+  }
+  after = Date.now();
+  console.log(`Done scoring (took ${after - before} ms)`);
+
+  // Finally, time to dump stuff.
+  const outputFilename = flags.get('output');
+
+  console.log(`Writing data to ${outputFilename}`);
+
+  const headers = ['date', 'sha', 'tests', ...products];
+  let data = headers.join(',') + '\n';
+
+  // ES6 maps iterate in insertion order, and we initially inserted in date
+  // order, so we can just iterate |dateToScores|.
+  for (const [date, {sha, scores, tests}] of dateToScores) {
+    const csvRecord = [
+      date.substr(0, 10),
+      sha.substr(0, 10),
+      tests,
+      ...scores,
+    ];
+    data += csvRecord.join(',') + '\n';
+  }
+  await fs.promises.writeFile(outputFilename, data, 'utf-8');
+}
+
+main().catch(reason => {
+  console.error(reason);
+  process.exit(1);
+});