This document provides an overview of Mastercard's Open APIs for developers. It begins with an explanation of basic credit card processing concepts. It then outlines several APIs available for issuers, acquirers, merchants and consumers. These include APIs for digital wallets, merchant checkout integration, fraud prevention, and more. Developers can access these APIs either through SDKs or directly via REST. The document concludes by discussing security considerations and providing tips for testing the APIs during hackathons.