This chapter summarizes multiple studies from a longitudinal database of 20 children from 21 to 33 months of age. First, we describe the growth of tense/agreement morpheme productivity, revealing an age-related starting point and stable individual growth trajectories. Next, we provide evidence for a developmental sequence characterized by -3s, -ed and auxiliary DO as a block of morphemes that develop together. Evidence for cross-morpheme facilitation is also presented. Finally, we document children's sensitivity to tense marking in parent input at 21 months of age, with input informativeness accounting for a significant portion of variation in between-child growth rates. We conclude that the acquisition of the tense/agreement system is the product of gradual morphosyntactic learning and that children have an abstract tense/agreement system before age three. Keywords: Tense/agreement; morphosyntax; morphosyntactic learning