This research assessed the value of case study methodology in the design of an educational computer simulation. Three sources of knowledge were compared to assess the value of case study: practitioner and programmer knowledge, disciplinary knowledge, and knowledge obtained from a case study of teacher practice. A retrospective analysis revealed that the case study was the source of 16 out of 23 design decisions and therefore was the most significant influence on the design of the simulation. The case study was particularly effective in sensitizing the design team to the classroom context, identifying common and uncommon teacher practices, revealing unexpected dimensions of interactions in the classroom, and generating constructive changes to the design of the simulation. Case studies can significantly enrich the design of educational software and are a promising methodological choice for design teams.