When designed well, application programming interfaces (APIs) for the open Web create a wide range of business opportunity. The problem that application development and delivery (AD&D) professionals must solve — what does "well-designed" mean? — applies not only to open Web APIs, but also to B2B APIs, internal APIs, and product APIs. The answer starts with clear consideration of each API's business context and intent, then moves on to API functionality, technology, and future agility. As a foundation for well-designed APIs, this report provides structured guidance on how to use business model, purpose, and architectural context as key pillars of an API design strategy.