To address the states' growing need for information about student performance in science, the Council of Chief State School Officers initiated the State Collaborative on Assessment and Student Standards (SCASS) Science Education Assessment Project. This multistate consortium pooled expertise and resources to apply standards and benchmark documents to develop state-of-the-art assessment tools and reform strategies to improve science education. This document outlines the parameters that guide SCASS assessment development. The assessment products, which may include portfolios, long-term investigations, hands-on activities, integrated essay questions, short constructed response items, and selected-response items are designed to collect data about what students know and can do in science. Facets that guide item development and ensure that exercises reflect relevant contextualized concepts begin with themes, unifying concepts that help link the disciplines of science. The four broad subject areas that guide the development of the assessment tools are life science, physical science, earth and space science, and coordinated science. The dimensions of knowing and inquiring about science through which depth of understanding is developed are acquiring, using, extending and knowing scientific knowledge. The overarching influences that help provide a relevant context for the assessments are historical and social and personal perspectives, the study of the nature of science, and the study of science and technology. (Contains three references.) (SLD)