This study evaluated the effects of a short-term professional development session. Forty volunteer high school biology teachers were randomly assigned to one of two professional development conditions: (a) developing deep content knowledge (i.e., control condition) or (b) evaluating student errors and understanding in writing samples (i.e., experimental condition). A pretest of content knowledge was administered, and then the participants in both conditions watched two hours of online videos about natural selection and attended different types of professional development sessions lasting four hours. The dependent variable measured teacher knowledge and skill related to evaluating student errors and understanding of natural selection. Significant differences between conditions in favor of the experimental condition were found on participant identification of critical elements of student understanding of natural selection and content knowledge related to natural selection. Results suggest that short-term professional development sessions focused on evaluating student errors and understanding can be effective at focusing a participant's evaluation of student work on particularly important elements of student understanding. Results have implications for understanding the types of knowledge necessary to effectively evaluate student work and for the design of professional development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]