Multitrait-multimethod (MTMM) confirmatory factor models were combined with longitudinal structural equation models to examine trait and method stability over time. A longitudinal correlated-trait correlated-method (CT-CM) model allowed for the study of trait and method variance in observed scores over time. Longitudinal measurement invariance was examined in the longitudinal CT-CM model to determine the invariance of the trait and method factors. The longitudinal MTMM model was then combined with second-order latent curve models to evaluate within-person change and between-person differences change in the trait factors while accounting for method-related variance. These models were developed and applied to longitudinal behavior-rating data from the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care and Youth Development with externalizing, internalizing, and social skills serving as the traits and mother, father, and teacher serving as methods or informants. Methodological extensions of longitudinal MTMM models and benefits of an MTMM approach to developmental research are discussed. (Contains 2 figures, 5 tables, and 2 footnotes.)