Delirium is a frequent complication in oncology. Its definition as a disorder of consciousness, attention, and cognition is useful to elaborate a rational framework of its pathophysiology and to interpret the role of different aetiological factors and therapeutic interventions. Many aetiologies and an interaction between risk and predisposing factors have been shown to contribute to most cases of delirium. A screening of potential aetiologies is always mandatory to benefit reversible cases. The palliative treatment of symptoms of delirium includes non-pharmacological, environmental, and preventive interventions and the use of haloperidol. If haloperidol fails to control delirium, sedation with other drugs can be necessary. Specific attention to the qualitative aspects of care and to the effect of delirium on family members should be given in the overall assessment of the patient in his or her cancer trajectory.