This is your hands-on guide to designing, building, and operating an API Marketplace to allow your organization to expose internal services and customer data securely for use by external developers. The book shows the mutual nature of a relationship in which organizations benefit from revenue and the reach of a new digital channel and third-party developers benefit from leveraging APIs to build unique applications. Providing open access is a regulatory requirement in some sectors, such as financial services, and this book helps you to build a platform to comply with regulatory requirements while at the same time encouraging and supporting use by external development teams. The book provides the blueprints for assembling teams and systems to build and support an API ecosystem. It offers insight into how the Marketplace can be constructed in a way to allow agility and flexibility to meet aggressive startup developer timelines while balancing established enterprise requirements of stability, reliability, and governance. The goal of this book is to provide engineering teams with a view of the operational requirements and how to meet and exceed these by establishing foundational elements at design time. An API Marketplace presents a unique challenge as organizations have to share internal capability and customer data with external developers. Security practices and industry standards are contrasted and discussed in this book. Practical approaches are provided to build and support a third-party developer ecosystem, manage sandbox environments hosting APIs of varying complexities, and cover monetization strategies that are yielding positive results to achieve self-sustainability.