Design is an inherently multidisciplinary endeavor. This raises the question of how to develop systems in ways that can best leverage the perspectives, practices, and knowledge bases of these different areas. Agile software development and usability engineering both address important aspects of system design, but there are tensions between the methods that make them difficult to integrate. This work presents a development approach that draws from extreme programming (XP), a widely practiced agile software development process, and scenariobased design (SBD), an established usability engineering process. It describes three key questions that need to be addressed for agile software development methods and usability engineering practices to work together effectively, and it introduces interface architectures and design representations that can address these questions.