Major misconceptions, or “alternative” conceptions as they’re sometimes called, and knowledge gaps about basic scientific concepts provide huge barriers to students of all ages as they acquire science knowledge. These have been the topic of many studies over the last 20 years though we continue to uncover new misconceptions that inhibit accurate construction of knowledge by our students. A number of research groups have been developing diagnostic items to help identify and correct knowledge gaps, for improving learning in specific classes and to improve core curriculua. We have been using diagnostic tests, in which incorrect responses (distracters) reflect common alternative conceptions, to help inform our curriculum for Biology majors at a major public university. The goal is to determine whether or not students have acquired a deep understanding of the biology concepts via their current course of study. The research team has developed diagnostic test items focusing on four key areas of naive conceptions: natural selection, metabolism (including respiration, photosynthesis and decomposition), osmosis and diffusion, and cell division. By assessing students at the beginning and end of several core courses, we use these surveys to measure gains in conceptual knowledge. Student scores on diagnostic tests are typically low (~35-45 points out of 100) because of the attractiveness of the distracters. Thus, student learning gains are reported more often than actual student scores. The most striking observation with diagnostic assessment is how consistently students continue to hold their naive ideas about science.