We are proposing and analyzing a cooperative prototyping approach wherein users are involved actively and creatively in design of computer applications. What we are suggesting is use of early prototypes in attempts to create and explore new work activities for the involved users. This chapter illustrates the approach by analyzing the design of computer support for casework in a technical department of a Danish municipality. Prototyping, as a design activity, is viewed as an ongoing learning process, and we analyze situations where openings for learning occur in the prototyping activity. Users and designers engage in this design activity, based on their different practices, and the development of the use activity is the primary focus of design. The situations that we are analyzing seem to fall into four categories:

1. Situations where the future work situation with a new computer application is simulated to investigate the future work activity.

2. Situations where the prototype is manipulated and used as a basis for idea exploration.

3. Situations focussing on the designers' learning about the users' work practice.

4. Situations where the prototyping tool or the design activity as such becomes the focus.

Lessons learned from the analysis of these situations are discussed. In particular, we discuss a tension between the need for careful preparation of prototyping activity and the need to establish conditions for user and designer creativity. Our approach to understanding these prototyping situations is inspired by activity theory, in particular the work of Engeström, wherein learning is seen as an expansion of work practice.