Instant Payment Notification (IPN) is PayPal's message service that sends a notification when a transaction is affected. Once IPN is integrated, sellers can automate their back office so they don’t have to wait for payments to come in to trigger order fulfillment.

IPN can send notifications for these transactions:

• Instant payments, including Express Checkout and direct credit card payments
• eCheck payments and pending, completed, or denied status payments
• Pending payments
• Recurring payments and subscriptions
• Authorizations
• Disputes, chargebacks, reversals, and refunds

You can also view notifications on PayPal’s IPN History page and resend them if you need to.

As PayPal’s interface for handling purchase confirmation and server-to-server communications, IPN can also be used to manage and customize a variety of APIs and communications, including:

• Customize your website's response to customer purchases in seconds
• Track customers via IPN "pass-through" variables
• Notify sellers who deal mostly in software downloads and other digital, online goods
• Track affiliate sales and commissions
• Store transaction information in your own database

For more about IPN:

• Technical Overview (link to new html version of docs on developer central)
• Instant Payment Notification Guide (PDF)
• Code Samples