This paper examines several issues and challenges associated with performance-based assessment in science through experiences with two large-scale student assessment projects. The first is the Common Core of Learning (CCL) Science Assessment Project designed to explore the use of classroom-embedded performance assessment for the evaluation of students by their own teachers. The second is the Connecticut Academic Performance Testing (CAPT) program, designed as a statewide, on-demand assessment of tenth graders' knowledge of science. On-demand assessment is external to classroom evaluation; the teacher has almost no input into its design, administration, and scoring. During the 4 years of the CCL project (1989-1993), approximately 200 science teachers developed and used performance tasks in their classes, but the use of performance assessments did not spread to large numbers of teachers until the statewide CAPT came into use. A survey of 34 school science supervisors indicated that the impact of the CAPT is most apparent in the laboratory and on open-ended test questions. Two tables and seven figures illustrate these discussions. (Contains 17 references.) (SLD)