Many mental disorders are associated with disrupted cognitive function including dysfunctions of attention, concentration and memory. Disorders of cognitive development in children and young people are problematic by themselves. We measure cognition using laboratory-based tests that can help us understand how it may be disrupted by mental disorders, as well as tell us something about the brain systems that might be involved. We also measure cognitive function in combination with direct measures of electromagnetic brain activity and with brain imaging methods, such as MRI. The study of cognition can help us understand, which brain mechanisms are involved in the symptoms and the possible treatment of illness. Some of our models are often analogous to computational programmes, specifying the computations that allow the brain to support functions including attention, memory, producing and understanding language, problem solving, and decision-making.