Since 2002, Project WHIRL (Wireless Handhelds In Reflection on Learning) has investigated potential uses of handheld computers in K–12 science classrooms using a teacher-involved process of software development and field trials. The project is a three-year research and development grant from the National Science Foundation, and it is a partnership between SRI International and a medium-sized district in South Carolina, Beaufort County School District. In contrast to many recent handheld development projects aimed at developing curricular materials, Project WHIRL focused on the development of assessment materials. In Project WHIRL, teachers were asked to apply their own curricular materials, content understanding, and pedagogical content knowledge to the project. Teachers and SRI researchers, software developers, and assessment specialists worked together to design software and activities that could be used across a variety of topic areas and science and in multiple phases of instruction to improve classroom assessment. This design process revealed to the research team teachers’ beliefs and assumptions about assessment as well as a wide range of practices they used to find out what their students know and can do, both informal and formal. In this paper, we focus on how teachers’ initial teaching and assessment practices influenced the design of handheld software and the ways in which these designs have been used across a variety of teachers’ classrooms. In addition, this paper provides some preliminary answers to two of the key research questions we outlined at the outset of our project:• What kinds of software designs can be feasibly implemented in classrooms that support effective assessment practice?• What are the conditions under which teachers can adopt handheld tools to support classroom assessment?