The momentum toward reform of science education brings pressures on schools and teachers to evaluate or otherwise account for children's progress in science. Although this interest can bring with it a certain amount of rush to judgment, it brings an opportunity to explore assessment alternatives that are fundamentally different from conventional evaluation methods. This paper focuses on one purpose of assessment, to inform instruction and support learning, starting from the premise that the foremost function of classroom assessment in the early years is to enhance teachers' powers of observation and understanding of children's learning. The paper discusses the guiding principles of preschool assessment: (1) including multiple forms and sources of evidence; and (2) using evidence collected over time, evidence highlighting what the individual knows, and evidence showing the collective knowledge of groups of learners. The paper also discusses documentation as an approach to assessment, including children's talk, guidelines for documenting science discussions, and a sample document recording a class discussion. The paper concludes with an examination of how lessons from early literacy assessment can be applied to early science assessment. (EV)