Evolution by natural selection provides the conceptual framework upon which much of modern biology is based: therefore understanding core ideas about biological evolution is an essential part of scientific literacy. Nonetheless, research repeatedly shows that high school and college students have difficulties understanding the notion of natural selection. To improve student learning, student progress at various points during instruction must be monitored. Assessment influences every level of the education system and therefore can serve as an important agent for change in science curriculum and instruction. Project 2061 of the American Association for the Advancement of Science has developed an analytical procedure to evaluate the quality of instructional materials, including their assessments, in terms of their likely contribution to the attainment of benchmarks and standards. Recently, this procedure has been applied to the evaluation of nine high school biology textbooks for their treatment of several topics including evolution. Findings indicate that most textbooks do not adequately assess students' understanding of evolution. This paper illustrates the range of good and poor assessment tasks included in currently available textbooks and considers implications for improving these tasks. (Contains 1 table and 1 box.)