diff --git a/src/__tests__/check-runner.test.ts b/src/__tests__/check-runner.test.ts
index 550146b..72df601 100644
--- a/src/__tests__/check-runner.test.ts
+++ b/src/__tests__/check-runner.test.ts
@@ -6,6 +6,12 @@ vi.mock('@/lib/checker', () => ({
   checkSite: vi.fn(),
 }))
 
+// Mock the notifications module
+const mockSendIncidentAlert = vi.fn()
+vi.mock('@/lib/notifications', () => ({
+  sendIncidentAlert: (...args: unknown[]) => mockSendIncidentAlert(...args),
+}))
+
 // Mock the admin client
 const mockFrom = vi.fn()
 const mockSupabase = { from: mockFrom }
@@ -20,6 +26,7 @@ import { checkSite } from '@/lib/checker'
 describe('POST /api/checks/run', () => {
   beforeEach(() => {
     vi.clearAllMocks()
+    mockSendIncidentAlert.mockResolvedValue(undefined)
     process.env.CRON_SECRET = 'test-secret'
   })
 
@@ -60,11 +67,9 @@ describe('POST /api/checks/run', () => {
     expect(body).toEqual({ message: 'No sites to check', checks: 0, incidents: 0 })
   })
 
-  test('records checks and opens incidents for failures', async () => {
+  test('records checks and opens incidents for failures, sends email', async () => {
     const site = { id: 'site-1', name: 'Test', url: 'https://example.com' }
 
-    // Mock sites query
-    const mockSelect = vi.fn()
     mockFrom.mockImplementation((table: string) => {
       if (table === 'sites') {
         return { select: () => ({ data: [site], error: null }) }
@@ -91,10 +96,21 @@ describe('POST /api/checks/run', () => {
               }),
             }),
           }),
-          insert: vi.fn().mockReturnValue({ data: null, error: null }),
+          insert: () => ({
+            select: () => ({
+              single: () => ({ data: { id: 'inc-1' } }),
+            }),
+          }),
         }
       }
-      return { select: mockSelect }
+      if (table === 'contacts') {
+        return {
+          select: () => ({
+            data: [{ email: 'alice@example.com' }],
+          }),
+        }
+      }
+      return {}
     })
 
     vi.mocked(checkSite).mockResolvedValue({
@@ -112,5 +128,63 @@ describe('POST /api/checks/run', () => {
     const body = await response.json()
     expect(body.checks).toBe(1)
     expect(body.incidents).toBe(1)
+    expect(mockSendIncidentAlert).toHaveBeenCalledWith({
+      siteName: 'Test',
+      siteUrl: 'https://example.com',
+      error: 'HTTP 503',
+      incidentId: 'inc-1',
+      contactEmails: ['alice@example.com'],
+    })
+  })
+
+  test('does not send email for existing open incident', async () => {
+    const site = { id: 'site-1', name: 'Test', url: 'https://example.com' }
+
+    mockFrom.mockImplementation((table: string) => {
+      if (table === 'sites') {
+        return { select: () => ({ data: [site], error: null }) }
+      }
+      if (table === 'checks') {
+        return {
+          insert: () => ({
+            select: () => ({
+              single: () => ({
+                data: { id: 'check-2', site_id: 'site-1', status: 'failure' },
+              }),
+            }),
+          }),
+        }
+      }
+      if (table === 'incidents') {
+        return {
+          select: () => ({
+            eq: () => ({
+              eq: () => ({
+                limit: () => ({
+                  single: () => ({ data: { id: 'inc-existing' } }),
+                }),
+              }),
+            }),
+          }),
+        }
+      }
+      return {}
+    })
+
+    vi.mocked(checkSite).mockResolvedValue({
+      status: 'failure',
+      statusCode: 503,
+      error: 'HTTP 503',
+    })
+
+    const request = new NextRequest('http://localhost/api/checks/run', {
+      method: 'POST',
+      headers: { authorization: 'Bearer test-secret' },
+    })
+
+    const response = await POST(request)
+    const body = await response.json()
+    expect(body.incidents).toBe(0)
+    expect(mockSendIncidentAlert).not.toHaveBeenCalled()
   })
 })
diff --git a/src/__tests__/notifications.test.ts b/src/__tests__/notifications.test.ts
new file mode 100644
index 0000000..81e36f7
--- /dev/null
+++ b/src/__tests__/notifications.test.ts
@@ -0,0 +1,82 @@
+import { describe, test, expect, vi, beforeEach } from "vitest"
+
+// Mock Resend
+const mockSend = vi.fn()
+vi.mock("resend", () => ({
+  Resend: class {
+    emails = { send: mockSend }
+  },
+}))
+
+import { sendIncidentAlert } from "@/lib/notifications"
+
+describe("sendIncidentAlert", () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    mockSend.mockResolvedValue({ id: "email-1" })
+  })
+
+  test("sends email to all contacts with correct content", async () => {
+    await sendIncidentAlert({
+      siteName: "Main Website",
+      siteUrl: "https://example.com",
+      error: "HTTP 503",
+      incidentId: "inc-1",
+      contactEmails: ["alice@example.com", "bob@example.com"],
+    })
+
+    expect(mockSend).toHaveBeenCalledTimes(1)
+    const call = mockSend.mock.calls[0][0]
+    expect(call.to).toEqual(["alice@example.com", "bob@example.com"])
+    expect(call.subject).toContain("Main Website")
+    expect(call.subject).toContain("[Down]")
+    expect(call.html).toContain("Main Website")
+    expect(call.html).toContain("https://example.com")
+    expect(call.html).toContain("HTTP 503")
+    expect(call.html).toContain("/incidents/inc-1")
+  })
+
+  test("does nothing when contactEmails is empty", async () => {
+    await sendIncidentAlert({
+      siteName: "Main Website",
+      siteUrl: "https://example.com",
+      error: "HTTP 503",
+      incidentId: "inc-1",
+      contactEmails: [],
+    })
+
+    expect(mockSend).not.toHaveBeenCalled()
+  })
+
+  test("handles email send failure gracefully", async () => {
+    const consoleSpy = vi.spyOn(console, "error").mockImplementation(() => {})
+    mockSend.mockRejectedValue(new Error("API rate limit"))
+
+    await sendIncidentAlert({
+      siteName: "Main Website",
+      siteUrl: "https://example.com",
+      error: "HTTP 503",
+      incidentId: "inc-1",
+      contactEmails: ["alice@example.com"],
+    })
+
+    expect(consoleSpy).toHaveBeenCalledWith(
+      "Failed to send incident alert email:",
+      expect.any(Error)
+    )
+    consoleSpy.mockRestore()
+  })
+
+  test("works without error field", async () => {
+    await sendIncidentAlert({
+      siteName: "Main Website",
+      siteUrl: "https://example.com",
+      error: null,
+      incidentId: "inc-1",
+      contactEmails: ["alice@example.com"],
+    })
+
+    const call = mockSend.mock.calls[0][0]
+    expect(call.html).not.toContain("<strong>Error:</strong>")
+  })
+})
diff --git a/src/app/api/checks/run/route.ts b/src/app/api/checks/run/route.ts
index 0fe636d..0e18aad 100644
--- a/src/app/api/checks/run/route.ts
+++ b/src/app/api/checks/run/route.ts
@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server'
 import { createAdminClient } from '@/lib/supabase/admin'
 import { checkSite } from '@/lib/checker'
+import { sendIncidentAlert } from '@/lib/notifications'
 
 export const maxDuration = 60
 
@@ -59,12 +60,30 @@ export async function POST(request: NextRequest) {
         .single()
 
       if (!openIncident) {
-        await supabase.from('incidents').insert({
-          site_id: site.id,
-          check_id: check.id,
-          status: 'open',
-        })
+        const { data: incident } = await supabase
+          .from('incidents')
+          .insert({
+            site_id: site.id,
+            check_id: check.id,
+            status: 'open',
+          })
+          .select('id')
+          .single()
         newIncidents++
+
+        if (incident) {
+          const { data: contacts } = await supabase
+            .from('contacts')
+            .select('email')
+
+          await sendIncidentAlert({
+            siteName: site.name,
+            siteUrl: site.url,
+            error,
+            incidentId: incident.id,
+            contactEmails: (contacts ?? []).map((c: { email: string }) => c.email),
+          })
+        }
       }
     }
   }
diff --git a/src/lib/notifications.ts b/src/lib/notifications.ts
new file mode 100644
index 0000000..f528caf
--- /dev/null
+++ b/src/lib/notifications.ts
@@ -0,0 +1,49 @@
+import { Resend } from "resend"
+
+let _resend: Resend | null = null
+function getResend() {
+  if (!_resend) _resend = new Resend(process.env.RESEND_API_KEY)
+  return _resend
+}
+
+type IncidentAlertParams = {
+  siteName: string
+  siteUrl: string
+  error: string | null
+  incidentId: string
+  contactEmails: string[]
+}
+
+export async function sendIncidentAlert({
+  siteName,
+  siteUrl,
+  error,
+  incidentId,
+  contactEmails,
+}: IncidentAlertParams) {
+  if (contactEmails.length === 0) return
+
+  const appUrl = process.env.NEXT_PUBLIC_APP_URL || "https://status.rootsofprogress.org"
+  const incidentLink = `${appUrl}/incidents/${incidentId}`
+
+  const subject = `[Down] ${siteName} is not responding`
+  const html = `
+    <h2>${siteName} is down</h2>
+    <p><strong>URL:</strong> ${siteUrl}</p>
+    ${error ? `<p><strong>Error:</strong> ${error}</p>` : ""}
+    <p><a href="${incidentLink}">View incident details</a></p>
+  `.trim()
+
+  const fromAddress = process.env.RESEND_FROM_EMAIL || "alerts@status.rootsofprogress.org"
+
+  try {
+    await getResend().emails.send({
+      from: fromAddress,
+      to: contactEmails,
+      subject,
+      html,
+    })
+  } catch (err) {
+    console.error("Failed to send incident alert email:", err)
+  }
+}