The neurotransmitter acetylcholine has been implicated in animal and human studies of delirium. This chapter will briefly review the clinical studies focussing on measurement of serum levels of anticholinergic activity in delirious states. Three approaches have been taken. First, to identify medications currently prescribed that have subtle anticholinergic effects. The current ‘list’ includes 48 commonly prescribed medications. Second, to associate serum anticholinergic activity with delirium in various clinical states including postcardiotomy delirium, postelectroconvulsive delirium, delirious elderly medical inpatients, and nursing home patients. Third, to intervene in patients with elevated anticholinergic activity by reducing known anticholinergics and correlating this reduction with clinical measures of cognition and delirium. Our most recent data investigates the impact of anticholinergics on demented patients. Rates of delirium were significantly higher in patients receiving larger numbers of anticholinergics.