The ETIPS software is a web-based learning environment that delivers cases that allow educators to practice instructional decision making. Here I recount its development but mainly emphasize the two key concepts that were central to our design process. The first was the Conceptual Assessment Framework, an evidentiary reasoning and design perspective that helped us to focus on which key attributes to build into the software and cases. The second concept is described as extreme programming, which is an iterative approach to software programming based upon user stories and rapid prototyping. The story of developing the ETIPS cases illustrates the need to know very clearly what the point is of the educational experience you are creating and to design software where form follows function.