Background/Question/Methods If ecology teaching is effective then students should correct their common misperceptions or popular misunderstandings. Collecting data on misconceptions can help assess teaching, while also providing a tool to engage students with course content as they confront their misunderstandings and compare their responses to the whole class. For two years in a college Ecology course, I examined pre- and post-test responses to questions about fundamental ecological misconceptions. Results/Conclusions An overwhelming majority of students entered the course with common misconceptions about populations, communities and ecosystems. Understanding was improved through the course, and most questions in post-tests were answered correctly by over 75 percent of students. However, some misconceptions were especially recalcitrant and difficult to change, with only 17-60 percent correct answers. Challenging topics included whether populations remain stable to maintain a “balance” of nature (No), whether decomposers release energy that is cycled back to plants (No), and the source of weight gain in plants (atmospheric CO2). We will discuss why these (and other) topics challenge students and how preconceptions interfere with understanding. This talk will also address strategies to assess teaching and learning as painlessly as possible.