Delirium is a common and under-reported problem in older adults. A lack of consensus over the terminology used to describe delirium may contribute to its underdiagnosis, and a number of predisposing factors are linked to it. This article, the first of two, aims to define delirium, describe the different types and identify risk factors associated with its development. Delirium is an important condition as it has a deleterious effect on patient outcomes and quality of life if it is not detected early and managed effectively. The detection of delirium and a variety of assessment tools are also considered. Part two, to be published in next week’s Nursing Standard, examines the nursing management of delirium.