Over the past two decades, mathematics education reformers have increasingly called for new approaches to assessment. This call urges mathematics teachers to merge their assessment with pedagogy, to employ assessments that engage students in the process of "doing mathematics," and to exploit assessment to support learning. This paper presents a research study of how six secondary mathematics preservice teachers learned to use such reform-based assessment practices while enrolled in one of three reform-minded teacher education programs. Data includes three interviews that concentrate on each preservice teacher's views on assessment. In addition, a series of classroom observations focus on the ways each preservice teacher assessed student learning. Analysis of the data revealed three distinct stages in the evolution of preservice teachers' assessment perspectives and practices--from traditional to more reform-based. Findings indicate that preservice teachers first focus on how to assess before considering other assessment functions such as what to assess and how to use assessment.