How can we understand why a bank teller has different needs for a user interface than those of casual users of a machine teller, or why a graphic designer needs a different user interface than a secretary? This article presents a framework for the design of user interfaces that originates from the work situations in which computer-based artifacts are used: The framework deals with the role of the user interface in purposeful human work. Human activity theory is used in this analysis. The purpose of this article is to make the reader curious and hopefully open his or her eyes to a somewhat different way of thinking about the user interface. The article applies examples of real-life interfaces to support this process, but it does not include a systematic presentation of empirical results. I focus on the role of the computer application in use. Thus, it is necessary to consider human-computer interaction and other related work conditions. I deal with human experience and competence as being rooted in the practice of the group that conducts the specific work activity. The main conclusions are: The user interface cannot be seen independently of the use activity (i.e., the professional, socially organized practice of the users and the material conditions for the activity, including the object of the activity). The standard view in these situations is to deduce an ultimate set of operations from an abstract use activity and apply these to design and analysis. This article argues that the user interface fully reveals itself to us only when in use. What is a good user interface for those with a certain degree of competence may not be efficient for those with different levels of competence. I give certain general recommendations for the user interface, but I have no guarantee that such recommendations are applicable to the specific case wherein these concerns may be overruled by specific social or material concerns.