Rather than a specific entity, delirium is at the midpoint on a spectrum of potential mental status changes that ranges from full consciousness to deep coma. The extremes are relatively easy to recognize, but other points along the spectrum may go unrecognized or be misdiagnosed. If recognized and treated expeditiously, delirium may be reversed in some patients. It is imperative that those caring for critically ill patients with cancer have the knowledge and tools necessary to identify and manage delirium appropriately. Although all critically ill patients are at risk for delirium, cancer presents additional assaults to the central nervous system via direct tumor invasion or iatrogenic provocations. This article describes delirium in cancer, and addresses diagnostic and management issues across the course of the disease.