Overall aim: To explore the impact of Parkinson's-related dementia (PRD) on the outcomes of life partners, and to investigate the effects of Parkinson's-adapted Cognitive Stimulation Therapy (CST-PD) on life partners. Background: Care partners play a crucial role in supporting people with PRD. The literature on the impact of PRD on care partners is vast; however, less is known about the impact of PRD on life partners who take on the additional role of a care partner. As care provision takes place within dyadic relationships, the outcomes of life partners may differ compared to non-life partners. Continuous care provision in PRD can lead to adverse physical and mental health outcomes in care partners but additionally, can take a toll on relationships in life partners. Long-term relationships are important as they can determine happiness, influence physical and mental health and lengthen one's lifespan but complex neurodegenerative conditions such as PRD may significantly disrupt dyadic relationships. Caregiving experiences in PRD, with a particular emphasis on long-term relationships, were examined in this thesis from the perspective of life partners. Methods: The objectives of the PhD were met through five related studies using different methodologies. First, with the people with PRD as the focus, the profile of life partners (n=136) was described. Second, in this same cohort, care burden was deconstructed using a factor analysis to ascertain the specific dimensions underlying the construct of burden. Third, the caregiving experiences of life partners were explored in detail using semi-structured qualitative interviews (n=12). Fourth, the associations among various health-related outcomes and relationship satisfaction were investigated in dyadic analysis with couples within PRD (n=57). Finally, the impact on life partners (n=57) of a dyadic intervention, CST-PD, was explored in a pilot randomised controlled trial. Results: Life partners of people with PRD were mostly married women who exhibited low levels of mental health and high levels of relationship dissatisfaction, burden, stress and negative feelings, predominantly once dementia had emerged in PD. Deconstructing burden further suggested a five-factor solution, contrary to expectation. In-depth qualitative interviews revealed that life partners experienced changes in the relationship including role transitioning, multiple care-related challenges, and loss of freedom and independence due to their caregiving role. At the same time, marital vows, acceptance, adjustment and resilience were important for life partners. The dyadic analysis demonstrated that health-related outcomes and relationship satisfaction were bidirectional and the anxiety of the person with PRD impacted on relationship satisfaction of both members of the couple. Finally, CST-PD increased positive interactions with the person with PRD but changes in other domains were not noted among life partners. Conclusions: This PhD makes a timely and valuable contribution to understanding the impact of PRD on life partners and illustrates that dyadic psychosocial interventions can be beneficial. It has important implications for future service provision for care partners. Given the benefit of including both members of the couple in the interventions, dyadic trials to support caregiving relationships, maintain quality of life and delay institutionalisation of people with PRD may be an important focus of activity in the future.