Acetylcholinesterase (AChE) predominates in the healthy brain, with butyrylcholinesterase (BuChE) considered to play a minor role in regulating brain acetylcholine (ACh) levels. However, BuChE activity progressively increases in patients with Alzheimer's disease (AD), while AChE activity remains unchanged or declines. Both enzymes therefore represent legitimate therapeutic targets for ameliorating the cholinergic deficit considered to be responsible for the declines in cognitive, behavioral and global functioning characteristic of AD. The two enzymes differ in substrate specificity, kinetics and activity in different brain regions. Experimental evidence from the use of agents with enhanced selectivity for BuChE (cymserine analogues, MF-8622) and the dual inhibitor of both AChE and BuChE, rivastigmine, indicates potential therapeutic benefits of inhibiting both AChE and BuChE in AD and related dementias. Recent evidence suggests that both AChE and BuChE may have roles in the aetiology and progression of AD beyond regulation of synaptic ACh levels. The development of specific BuChE inhibitors and further experience with the dual enzyme inhibitor rivastigmine will improve understanding of the aetiology of AD and should lead to a wider variety of potent treatment options.