Background: Cognitive decline is commonly reported in Parkinson’s disease (PD), with some deficits evident even at the onset of PD. Executive functions (EF) are extensively studied in PD and emerge as the domain involving the most profound deficits. Nevertheless, there are some inconsistencies in the literature with regard to the exact pattern of executive deficits and their impact on everyday life in PD. The aim of the literature review presented in this thesis was to synthesise and clarify existing research evidence on EF in early stage PD, and to explore what are the possible factors affecting the consistency of research findings. The empirical studies had three distinct aims: to clarify the pattern of EF deficits in PD; to determine how accurately PwPD appraise potential EF-related difficulties; and to identify how executive deficits impact on people with PD (PwPD) and their families. Method: Studies of EF in PD were systematically reviewed and the findings were synthesised in a series of meta-analyses. Three empirical studies drew on cross-sectional data collected from PwPD and their caregivers, and from healthy older controls. Sixty-five PwPD in mild to moderate stages of PD completed an assessment of EF, awareness, quality of life, and health status, and 43 healthy older controls completed assessment of EF and awareness. Fifty caregivers of PwPD rated the EF of the PwPD and their own burden associated with caring for a PwPD. A sub-group of 34 PwPD, identified as having potential EF deficits, completed a more extensive neuropsychological assessment of executive abilities. Results: The systematic review included 33 studies of EF in early stage PD, and metaanalysis of data from 5 commonly-used tests of EF revealed consistent evidence for executive deficits. The review suggested that the consistency of the research evidence may be improved by more precision in defining EF and more careful selection and interpretation of EF measures. A data-driven analysis examining the pattern of EF impairment distinguished differences between two groups of standard tests of EF, with attentional control tests more frequently compromised than abstract thinking in early stage PD. PwPD were found to be accurate when making general evaluative judgments about their own functioning, but in specific tasks PwPD with executive deficits overestimated their performance in comparison to PwPD without EF deficits and healthy controls. EF-related behavioural difficulties were shown to impact on subjective quality of life in PwPD and on burden in their caregivers. Conclusions: The results of this thesis suggest that EF-related difficulties are frequently present in early stage PD, with attentional control aspects of EF particularly affected, that it may be difficult for PwPD to accurately appraise their own ability to carry out specific activities, and that EF-related difficulties have a significant impact on quality of life in PwPD and their families. A thorough understanding of executive deficits in PD is important in the provision of adequate person-centred care for PwPD and their family members, and could help to inform the development of PD-specific rehabilitative interventions aimed at reducing activity limitation and restrictions on social participation and supporting PwPD in living well with the condition.