This book outlines the new concept of user engineering and covers the diversity of users, along with the business process that includes the design and the user's experience processes. Although the concept of user experience (UX) has become popular, the definition and the methodology are still ambiguous. User engineering is similar to the user-centered design, but differs in that its scope is not limited to the design process but concerns the whole manufacturing process and the whole usage process, i.e., the whole lifecycle of an artifact. User's perspective is strongly emphasized in this book, hence, its stance is far from that of the marketing approach that usually fails to notice the life and experiences of users after the purchase of an artifact as consumers. Theory of User Engineering differentiates between the quality in design and the quality in use, and the objective quality characteristics and the subjective quality characteristics. In addition to the user research using ethnographic methods, the author introduces a new approach based on the artifact evolution theory that can be adopted in the planning stage.