The field of astronomy education is rapidly growing beyond merely sharing effective activities or curriculum ideas. This paper categorizes and summarizes the literature in astronomy education research and contains more than 100 references to articles, books, and Web-based materials. Research into student understanding on a variety of topics now occupies a large part of the literature. Topics include the shape of Earth and gravity, lunar phases, seasons, astrobiology, and cosmology. The effectiveness of instructional methods is now being tested systematically, taking data beyond the anecdotal with powerful research designs and statistical analyses. Quantitative, qualitative, and mixed- methods approaches have found their places in the researcher’s toolbox. In all cases, the connection between the research performed and its effect on classroom instruction is largely lacking.