Responsiveness defines the prompt, contingent, and appropriate reactions parents display to their children in the context of everyday exchanges. Maternal responsiveness occupies a theoretically central position in developmental science and possesses meaningful predictive validity over diverse domains of children's development, yet basic psychometric features of maternal responsiveness are still poorly understood. In this prospective longitudinal study, the authors examined structure, individual variation, and continuity of multiple dimensions of responsiveness in 40 mothers to their infants' activities at 10, 14, and 21 months during natural home-based play interactions. Both age-general and age-specific patterns emerged in maternal responding. The study's developmental results support the multidimensionality, modularity, and specificity of this central parenting construct. (Contains 4 tables and 4 figures.)