Although it comprises only 2 percent of the lifespan, infancy is one of the most remarkable and busiest times of development. The newborn baby enters the world with surprisingly sophisticated perceptual and motor abilities, a set of skills for interacting with people, and a capacity to learn that is put to use immediately after birth.

Reflexes are the newborn's most obvious organized patterns of behavior. Some have survival value; others form the basis for motor skills that will develop later. Infants move in and out of six states of arousal that become more organized and predictable with age. Like children and adults, infants alternate between REM and NREM sleep, although they spend far more time in the REM state than they ever will again. Young infants are believed to have a special need for REM sleep; their brain-wave activity safeguards the central nervous system, and the rapid eye movements protect the health of the eye. Crying is the first way that babies communicate, letting parents know they need food, comfort, and stimulation. An infant's cry stimulates strong feelings of arousal and discomfort in almost everyone, and controversy exists on how quickly and how often parents should respond. Abnormal crying may indicate central nervous system distress.

Babies come into the world with a built-in set of learning mechanisms that permit them to profit from experience immediately. Infants are capable of two basic forms of learning: classical and operant conditioning. Habituation and dishabituation research provides a window into infant attention, perception, and cognition, and reveals that infants are naturally attracted to novel stimulation and that their recognition memory improves steadily with age. Newborn infants also have a remarkable capacity to imitate the facial expressions and gestures of adults.

According to dynamic systems theory, motor development is energized by the baby's exploration and desire to master new tasks and jointly influenced by central nervous system maturation, movement possibilities of the body, and environmental supports. Voluntary reaching plays a vital role in infant cognitive development and is integrated into increasingly elaborate motor skills. Newborns have a built-in sense of balance that is refined with experience and motor control, and their postural adjustments to self-movement take place unconsciously.

Sensitivity to touch, taste, smell, and sound are well developed in the newborn, and vision improves during the early months. Research on infants with severe visual impairments dramatically illustrates the interdependence of vision, social interaction, motor exploration, and understanding of the world. Depth perception develops as infants detect kinetic, binocular, and pictorial cues, and gradually babies move from focusing on the parts of a pattern to perceiving it as an organized whole. The Gibsons' differentiation theory provides an overall account of perceptual development.

Research findings on the question of whether infancy is a sensitive period of development indicate that early experience combines with current conditions to affect the child's development.