Although science fairs have been an institution of science education for decades in schools across the United States, little is understood about how students' science fair experiences vary and how these variations relate to student learning. Research on this topic is particularly imperative as new science standards increase emphasis on the teaching of science and engineering practices. Science fairs represent a potential opportunity to engage students in these practices, but may not be effective in supporting the learning of all students. As a first step in a programmatic research agenda, this study employs a nationally representative survey of middle schools to describe the most commonly conducted, broad approaches to middle school science fairs. Using a framework based on teacher support for inquiry, three types of science fairs emerged: mandatory fairs with high levels of teacher support for students' project work, mandatory fairs with low levels of teacher support, and voluntary fairs. Mandatory fairs with limited teacher support were more common in schools with a high proportion of African American students and high poverty, but were also more likely to emphasize goals related to learning. Implications for the effectiveness and equity of science fairs are discussed.