Moderate to severe Alzheimer's disease (AD) is characterized by increasing cognitive, functional, and behavioural dysfunction that results in increased caregiver burden and, eventually, complete dependence. Despite its significance as a societal health problem, there are few treatment trials of cognitive enhancers or disease modifying agents for this stage of illness. Studies suggest the cholinesterase inhibitors, especially donepezil, may provide benefit. Several studies provide support for the use of the NMDA receptor antagonist memantine as monotherapy or added to a cholinesterase inhibitor for moderate to severe AD. While there are no published guidelines for the treatment of moderate to severe AD, these studies do provide guidance for recommendations for study design and outcome measures. Such studies are urgently needed.