This chapter is about cognitive and perceptual development form age zero to three, focusing on three themes that change a young child: attention, memory, and understanding of the physical world through numbers, magnitude, and vision and perception. Observation and play are the work of infancy and are how babies learn about the world. Jean Piaget was a stage theorist who believed that we move through different stages of development. From zero to 18 months is the sensorimotor period of development, and then from 18 months to 2 years children use language as a symbol system and develop object permanence. Renee Baillargeon challenged this and said that babies look longer to the impossible event than the possible event, and Karen Wynn design an experiment that suggests that babies can do very rudimentary simple math. Researchers have used infants’ propensity towards gestures as a way to intervene and promote language development, in what is called “baby signs.”