Donepezil is a reversible, non-competitive piperidine-type acetylcholinesterase inhibitor (AChEI) that is structurally unique compared with other currently available AChEIs. It was developed as a symptomatic treatment to compensate for the progressive loss of cholinergic signal between neurons, a consequence of neuronal cell loss in the brains of patients with Alzheimer's disease (AD). Clinical trials conducted over the 15 years since the drug was first licensed and available for use in patients with mild to moderate AD (1997) have shown that donepezil is efficacious and well tolerated at all stages of AD, from patients with mild or moderate impairment to those with severe disease. The published literature contains nearly 2000 papers on donepezil, and more than 200 of these articles relate to randomized controlled clinical trials. Trials in patients with mild cognitive impairment (MCI) failed to meet all of their primary objectives, but provided insights into patient selection and the design of trials. Overall, more than a decade of donepezil research has gradually changed attitudes toward therapy of AD from a general belief that no clinically useful treatment exists to the present day understanding that symptomatic therapy may be effective across the spectrum of dementia stages.