This paper describes the task analysis of performance-based science tasks that were designed for the 1994 National Assessment of Educational Progress (NAEP) science assessment, now postponed until 1996, and field tested in 1993. A brief description of the science performance tasks is followed by a description of the task analyses performed and a discussion of the various uses of task analysis. In 1993, 17 performance-based tasks were field tested, with 5 for grade 4, 2 for grades 4 through 8, 3 for grade 8 only, 2 for grades 8 through 12, and 5 for grade 12 alone. In level one task analysis, the unit of analysis was the task as a whole. Analysis methods included observations, the think aloud method, questionnaires, and expert opinions. In level two analysis, the cognitive operations and skills of steps in the task were analyzed. As results in these field tests demonstrated, level one analysis results in a mapping of the task's structure from the perspectives of test takers. Level two analysis shows the nature of the skills for completing the task. Together, the levels can ascertain the extent to which tasks meet standards of validity. (Contains 3 tables, 2 figures, and 13 references.) (SLD)