The role of cognition in child development is reviewed with respect to the role of cognitive processing and cognitive deficits. Cognitive processing is discussed with respect to the self-system; the effects of psychosocial experiences; risk, vulnerability and protective mechanisms; vulnerability to depression; aetiology and treatment of depression. Cognitive deficits are discussed with respect to the socio-emotional consequences of language delay and reading difficulties; hyperkinetic/attentional deficit syndromes; schizophrenia; and autism. It is concluded that the ways in which we appraise our life circumstances and the ways in which we react to experiences of all kinds are greatly influenced by how we think about ourselves and our environment. Biases and distortions in such cognitive processing may be associated with social and emotional malfunction. These biases may derive from earlier experiences, from intensive temperamental styles, or from deficits in the ability to process incoming information. The further study of cognitive processing and cognitive deficits is likely to be rewarding and helpful for clinical practice.