Delirium is increasingly being addressed in healthcare settings as a commonly occurring condition that is serious, preventable, treatable, and often unrecognized. In addition, it can have serious consequences—including death—when it is not recognized and addressed. Older adults with dementia have the double disadvantage of having a higher incidence of delirium and having it overlooked as a separate and treatable condition. This chapter provides the definition and types of delirium. It discusses the predisposing and precipitating conditions associated with delirium. The chapter explains the consequences associated with delirium and delirium superimposed on dementia. It describes the roles of nurses in identifying conditions associated with delirium and the roles of nurses in addressing the needs of patients with delirium.