Abstract Temperament is defined as constitutionally based individual differences in reactivity and self-regulation. Temperament describes the initial state from which personality develops and is essential to understanding the origins of personality development. The infant’s brain is not a blank slate at birth to be shaped by parenting; children differ from infancy and afterwards in their temperament. Neurobiologically, temperament links individual differences in behavior to underlying neural networks. This chapter reviews the role of temperament and personality in socioemotional development from a developmental perspective. The chapter includes a historical introduction to temperament and a review of various definitions of temperament. It addresses the effects of temperament on others, the neurobiology of temperament, temperament and cognition, and the assessment of temperament in children with developmental disabilities.