Delirium is a common form of acute brain-dysfunction in critically ill patients, occurring in 30% to 80% of critically ill patients and is associated with a threefold increase in mortality risk and strongly associates with cognitive decline after intensive care unit (ICU) stay. A recent international guideline (2013) has therefore proposed to perform screening for delirium symptoms in all critically ill patients. However, both these international guidelines and national (Dutch) guidelines on ICU delirium have indicated that, in spite of the widespread use of haloperidol in critically ill patients, there are no adequately powered trials on efficacy of haloperidol for the treatment of delirium and associated adverse outcomes (mortality and cognitive decline). Therefore, we aim to undertake a prospective randomized double-blind placebo controlled clinical trial to assess: 1) the efficacy of haloperidol to reduce delirium in adult critically ill patients and thereby to 2) reduce long-term cognitive and functional impairment, unpleasant patient and family experiences and psychological sequelae after ICU stay and mortality and assess 3) safety concerns associated with haloperidol use. A health technology assessment will be performed from a healthcare- and societal perspective, e.g. accounting for long-term cognitive impairment.