This book is an account of how I addressed the need for a smartphone app that would allow someone with Type 1 diabetes to self-manage their condition. Its presentation highlights the major features of the app interface design. They include the selection of metaphors appropriate to a user need to form a mental model of the app; the importance of visible context; the benefits of consistency; and considerations of a user cognitive and perceptual abilities. The latter is a key feature of the book. But the book is also about the design process, and especially about the valuable contributions made by the many focus group meetings in which design ideas were first presented to people with Type 1 diabetes. Their critique, and sometimes their rejection, of interface ideas were crucial to the development of the app. I hope this book will prove useful for teaching and design guidance.