Measures of socioeconomic status (SES) are widely used in educational research and policy applications, in large part due to overwhelming evidence linking SES to student achievement. SES is usually conceptualized as an unobservable factor--a construct--measured using variables such as parental education, occupation, income/wealth, and home possessions to take into account disparities between students, classrooms, and schools. Researchers and policymakers agree on the importance of SES in educational settings, but their analyses typically rely on an invalid measure of it: eligibility for free and reduced-price lunch (FRL). This, along with other problems with FRL and other commonly used measures of SES, can call into question the validity of their conclusions. The brief offers several recommendations for how researchers can improve the measures they use for SES, and thereby promote both a deeper understanding and more effective use of SES in education research and policy.