User profiles or user models are vital in many areas in which it is essential to obtain knowledge about users of software applications. Examples of these areas are intelligent agents, adaptive systems, intelligent tutoring systems, recommender systems, intelligent e-commerce applications, and knowledge management systems. In this chapter we study the main issues regarding user profiles from the perspectives of these research fields. We examine what information constitutes a user profile; how the user profile is represented; how the user profile is acquired and built; and how the profile information is used. We also discuss some challenges and future trends in the intelligent user profiling area.