Concern about the education system has increasingly focused on achievement outcomes and the role of assessment in school performance. Our research with fifth and eighth graders in California explored several issues regarding student performance and rater reliability on hands-on tasks that were administered as part of a field test of a statewide assessment program in science. This research found that raters can produce reliable scores for hands-on tests of science performance. However, the reliability of performance test scores per hour of testing time is quite low relative to multiple-choice tests. Reliability can be improved substantially by adding more tasks (and testing time). Using more than one rater per task produces only a very small improvement in the reliability of a student's total score across tasks. These results were consistent across both grade levels, and they echo the findings of past research.