The study of the neurochemical modulation of memory has dominated the cognitive psychopharmacological literature, in both humans and animals, in recent years. This chapter concentrates on drug treatments for dementia, since it has been the driving force behind the search for cognitive enhancers. From the results of studies in both animals and humans, the cholinergic system has been the neurotransitter system most traditionally associated with the processes of memory and learning. Evidence of 'subcortical' dementia is noted in patients with Parkinson's disease (PD), where the principal lesion appears to be in the substantia nigra, leading to degeneration of the nigrostriatal dopamine system. The cholinergic hypothesis of Alzheimer's disease (AD) is one of the most enduring and resilient theories of cognitive psychopharmacology. There is accumulating evidence that the serotonergic system is compromised in patients with DAT.