This project is developing a basic Biology Concept Inventory (BCI) that will enable the field to reliably quantify student learning at the introductory college level. The objective is to arm the field with reliable data on student learning as biology departments around the country attempt to improve student achievement. The project is inspired, and will follow the development strategy, of similar efforts in Physics and Astronomy, which have been highly successful in significantly improving teaching in these fields. The development of valid, reliable instruments relies critically on the identification of the dominant misconceptions students carry into the classroom with them. Misconceptions are incorrect mental models of physical phenomena and processes that students hold before instruction. They present a significant barrier to learning, and they are best addressed explicitly with specifically designed learning activities. Once these misconceptions have been identified, an experienced team of content experts and evaluators can, through repeated review and test cycles, develop valid instruments that can reliably diagnose students' misconceptions. These instruments can then form the basis of course and curriculum transformation efforts that aim to improve student achievement. Development and assessment of the instrument involves collecting data on misconceptions (essays, questionnaires, and interviews) from over 2000 students in four schools, including a significant population of pre-service teachers. Intellectual Merit: The proposed project is using well-tested methodology to develop highly needed evaluation capacity for biology. Broader Impact: The existence of a Biology Concept Inventory has the potential to impact the teaching of biology to thousands of undergraduates throughout the country, in the same way that the Force Concept Inventory and the Astronomy Diagnostic Test have impacted the teaching of Physics and Astronomy.