No clear general view has emerged from the many recent studies devoted to cognitive disorders in Parkinson's disease. Disparity of the populations and methods, as well as interference from motor disorders and possible psychiatric disorders account for the variability of epidemiological data. Subtle cognitive disorders can be found in most patients at the very beginning of the disease. Recent memory is disturbed in free recall tests, but recognition capacities are preserved. Individualization of visuospatial disorders is discussed. Alteration of conceptual capacities is the predominant disorder. Clinically obvious cognitive disorders appear only in some patients after several years of neurological disease. Some risk factors are agreed upon. The bradyphrenia concept is debated, and the term dementia is not accepted by all authors. The major cognitive alterations in Parkinson's disease differ from those observed in Alzheimer dementia, but the latter is more frequent in parkinsonian patients than in the general population. Patients with Parkinson's disease are also exposed to the risk of transient psychotic episodes. Where parkinsonian cognitive pathology is concerned, modern imaging methods are of no help to clinicians. The anatomical and biochemical changes observed in these patients are reviewed, and their responsibility in the genesis of cognitive disorders is discussed: lesions of brainstem nuclei and alterations in their cortical projections on one side, Alzheimer type dementia lesions and Lewys' bodies on the other side. The diverse and inconstant cognitive disorders of patients with Parkinson's disease cannot be explained by lesions of one single structure; they probably result from variable combinations of multiple known or unknown neuronal and biochemical changes.