Digital tools cannot be separated from their users, the activities for which they are built, and their utilisation context. They require from professionals a learning of how to work with systems that are more or less easy to use, and a modification, sometimes complete, of their practices and organisations. It is why this chapter introduces a human factor approach of medical informatics, and specifically a work analysis approach, which is a methodological approach aiming to collect essential data in order to describe the psychological, physical, social, technical and economic conditions within which an operator performs a set of tasks or activities that constituted his work. After presenting the overall context of computerisation pressure associated with the evolution of health work systems and the possible risks and stakes it brings, this chapter describes the concepts of ergonomics and human factors for medical informatics projects. It specifically focuses on work situation analysis, user-centered design with a practical example concerning the implementation of a CPO system, and concludes with a broader approach opening the concept of medical socio-technical system.