The sensorimotor stage lasts from birth to about 2 years of age. In this stage, infants construct an understanding of the world by coordinating sensory experiences (such as seeing and hearing) with physical, motoric actions—hence the term “sensorimotor.” At the beginning of this stage, newborns have little more than reflexive patterns with which to work. At the end of the sensorimotor stage, 2-year-olds can produce complex sensorimotor patterns and use primitive symbols.

By the end of the sensorimotor period, children understand that objects are both separate from the self and permanent. Object permanence is the understanding that objects and events continue to exist even when they cannot be seen, heard, or touched.

Piaget divided the sensorimotor stage into six sub-stages:

1. Simple reflexes (Birth - 1 month): Coordination of sensation and action through reflexive behaviors
2. First habits and primary circular reactions (1- 4 months): Coordination of sensation and two types of schemas - habits (reflex) and primary circular reactions (reproduction of an event that initially occurred by chance). Main focus is still on infant's body.
3. Secondary Circular reactions (4-8 months): Infants move beyond self-preoccupation and become more object-oriented.
4. Coordination of secondary circular reactions (8-12 months): Coordination of vision and touch, schemes and intentionality.
5. Tertiary circular reactions, novelty and curiosity (12-18 months): Infants become intrigued by objects and experiments with new behavior,
6. Internalization of schemes (18-24 months): Infants develop the ability to use primitive symbols and form enduring mental representations.

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Sources:
Life-Span Development, John W. Santrock (Click for eBook)