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Background: Familial hypercholesterolaemia (FH) is a genetic disorder characterised by elevated low-density lipoprotein cholesterol (LDL-C) levels and increased risk of cardiovascular disease (CVD). Clinical guidelines advocate dietary and physical activity recommendations adjuvant to life-long pharmacological treatment. However, there is a lack of evidence to support the effectiveness of diet and physical activity in the treatment of FH. Aim: The aim of this thesis was to develop and evaluate an intervention targeting the dietary and physical activity behaviours of children and their parent who are affected by FH. Following the Medical Research Council guidance for the development and evaluation of complex interventions, this was addressed through four studies. Methods and results: Study one summarised the existing evidence base for the role of diet and physical activity in the treatment of FH. Study two synthesised the qualitative evidence regarding the experiences and beliefs of individuals with FH and identified several enablers and barriers to treatment adherence. This informed suggestions for clinical practice that could facilitate optimal treatment adherence. In study three, a theoretical framework was applied to the findings of study two to develop an intervention targeting dietary and physical activity behaviours of individuals with FH. Twenty-six behaviour change techniques were identified for inclusion in the intervention to target the theoretical constructs identified to influence behaviour. Study four was a randomised, controlled feasibility trial (n=21 families) that found it is feasible to recruit, randomise and retain families with FH and to implement and evaluate the intervention designed in study two. The results also provide evidence that the intervention may be associated with improvements in CVD risk factors, including LDL-C. Conclusion: The intervention development and evaluation undertaken for this thesis has provided the evidence required ahead of conducting a definitive randomised controlled trial (RCT). This RCT will be adequately powered to evaluate the potential effectiveness of diet and physical activity treatment recommendations on LDL-C treatment goals and CVD risk amongst individuals with FH.