The use of eligibility for a free lunch as a measure of a student's socioeconomic status continues to be a fixture of quantitative education research. Despite its popularity, it is unclear that education researchers are familiar with what student eligibility for a free lunch does (and does not) represent. The authors examine the National School Lunch Program, which is responsible for certifying students as eligible for a free lunch, and conclude that free lunch eligibility is a poor measure of socioeconomic status, which suffers from important deficiencies that can bias inferences. A table characterizing key strengths and weaknesses of variables used as measures of socioeconomic status is provided to facilitate comparisons. (Contains 2 figures and 3 tables.)