Chapter 6: User Interfaces for Mobile Media 201 Figure 6-1. The main components of the interactive system. A mobile device can be considered an interactive system that aids the user in accomplishing their goals. The term interaction refers to communication between the system and the user through an interface, composed of channels for input and output (Figure 6-1), usually referred as the user interface. Interaction is based on different human commu- nication channels that we discuss briefl y in section 6.1 (Dix et al. 2003). This chapter discusses some general characteristics of interactive systems, focusing on mobile aspects of user interfaces targeted at per- sonal content experience. We employ a number of in-house UI design concepts to illustrate the topics discussed. Each component of an interactive system corresponds to a transla- tion from a component to another during one interactive cycle. The cycle starts in execution phase, when the user formulates a task for achieving a goal and starts manipulating the system through input. As a result, the system transforms itself, based on the operations translated from the input. The system is in a new state that needs to be commu- nicated to the user during the evaluation phase. Therefore, the current values of system attributes are presented to the user. Then, it is the user’s task to observe the output and evaluate the results of interaction in relation to the original goal (Dix et al. 2003). Nonetheless, users do not live in a vacuum: a mobile device is operated in a certain context that affects its use to a great extent. Prior to discussing users, interactions with personal content, and mobile user interfaces, we will briefl y review the evolution of the user interfaces for computing devices. In the early 1950s, at the dawn of the computing era, use was based on punch cards that were fed into readers, the output then printed on paper. As such, the user could not interact with the system directly, which hindered its use in many ways. The beginning of the 1960s introduced command language user interfaces, as fi rst keyboards and alphanumeric displays became available. This era bloomed until the