It is critical to the success of a system that appropriate and representative users are involved in the development work. However, the process of identifying and selecting users has not been a focus of research. In this paper, we describe a process of identifying and selecting relevant users specifically for field studies. Characterization and sampling of users are discussed. In addition, a way of describing user groups is suggested. Finally, we summarize results from seven case studies in six different companies. The case studies show that developers tend to underestimate the diversity of users and that a systematic process helps to identify the different user groups, select representative users and identify representative user needs. It was found that identification of users' characteristics and groups is an iterative process, and user descriptions evolve when real data are gathered from users.