Motor skills are critical for development. Everything infants do involves motor skills—basic postural, manual, and locomotor actions, exploration, social interactions, and actions involving everyday artifacts. Put another way, all behavior is motor behavior, and thus motor skill acquisition is synonymous with behavioral development. Because motor behavior is observable, age norms for basic motor skills provide useful diagnostics for healthy versus atypical development. Motor skills lay the foundation for development by opening up new opportunities for learning. Postural control brings new parts of the environment into view and into reach, and independent mobility brings the larger world into play. Manual skills promote new forms of interactions with objects. And motor skills involving every part of the body enhance opportunities for social interaction. Thus, motor skills instigate a cascade of developments in domains far afield from motor behavior—facilitating developments in perception and cognition, language and communication, emotional expression and regulation, and social cognition. Finally, motor skill acquisition makes behavior increasingly functional and flexible. Infants learn to tailor behavior to variations in their body and environment and to discover or construct new means to achieve their goals.