The Strengths and Difficulties Questionnaire (SDQ) has been widely used to study children's psychosocial development longitudinally; however, such analyses assume longitudinal measurement invariance, that is, they presuppose that symptom manifestations are measured comparably across different ages. Violations of this assumption could bias longitudinal analyses and should therefore be empirically tested. This study tested longitudinal measurement invariance within a confirmatory factor analysis framework in the U.K.-based Avon Longitudinal Study of Parents and Children (N = 13,988). Results indicated that SDQ scores showed configural, metric, scalar, and residual invariance across ages 7, 8, 9, 11, 13, and 16, supporting its use for comparing variances, covariances, and means over time within a latent variable model as well as using observed scores. At age 4, configural invariance was not supported, indicating that mental health symptoms as measured by the SDQ manifest differently at this age, thus necessitating caution when comparing symptoms as measured by SDQ scores at this age to later ages.