The present work presents a series of studies investigating neuropsychological aspects of visual hallucinations (VHs) in Parkinson's Disease (PD) and high proneness to VHs in the normal population. The aim of the thesis is to investigate whether the same risk factors are implicated in both hallucinating PD patients and in high-prone individuals from the normal population, i.e. the continuum hypothesis of VHs. To this end, new instruments were designed to assess the nature of VHs in PD and to differentiate among , high and low hallucination-prone individuals. PD patients with and without VHs, agematched normal controls, and high and low-prone normal young individuals are assessed on visual memory and executive tests from the CANTAB test battery, alongside tests examining personality factors, sleep patterns, and demographic factors. The findings suggest that VHs in PD and hallucination-proneness in the normal population are both associated with a combination of different factors, particularly aspects of visual processing and sleep patterns. Results from the five studies are interpreted with the multifactorial models of VHs, suggesting that both VHs in PD and hallucinationproneness in the normal population stem from concurrent neuropsychological dysfunctions of several processing systems. However, a specific personality profile is predictive of high hallucination-proneness in the normal population, but not in PD patients. Therefore, two different models are proposed, arguing for similar, but not identical set of risk factors in hallucinating PD patients and in high-prone normal individuals.