Individuals with poor quality diets face an increased risk of developing a range of chronic diseases. Aspects of the local food environment, including food availability, have been linked to less healthy diets and increased body weight in adults. However, there is limited theory development to guide the synthesis of empirical studies or the design and evaluation of intervention strategies. Therefore, this dissertation sought to 1) develop a theory of change for unpacking the association between local food availability, diet quality and weight status through identifying hypothesized pathways of influence; and 2) test identified mechanisms using nationally representative data for adults across the United Kingdom and England. Firstly, a systematic review and realist synthesis were used to generate a novel theory of change for food availability, diet and obesity. This involved integration of tacit knowledge from an expert panel with evidence from a range of published food availability interventions. The resulting theory suggested that the influence of food availability on diet and weight status involves a complex set of pathways. Including the importance of understanding the link between the adoption of, and exposure to, different types of food outlets and the alignment of these factors with the preferences and needs of people of varying socioeconomic position (SEP). Thus, use of different away-from-home food outlets (i.e. fast food, restaurant and café) were examined in adults from the National Diet and Nutrition Survey showing that only use of fast food outlets (but not restaurants or cafés) was associated with poor diet quality and obesity, after accounting for SEP. Next, given the potential importance of exposure to the density of food outlets in their use, the same types of away-from-home food outlets were examined in a cross-sectional spatial study of adults from the first wave of the UK Household Longitudinal Study (UKHLS). This analysis showed that again, regardless of individual SEP, the density of fast food outlets around the home was positively associated with away-from-home food spending and obesity, while the density of restaurants was negatively associated with obesity and no link to the density of cafés. Lastly, preliminary longitudinal analysis using adults from five waves of the UKHLS explored the utility of residential relocation as a means of examining the effect of a change in exposure to away-from-home food outlet density on diet and obesity. Results suggest that relocation can lead to substantial change in food outlet exposures; however, relocation is also accompanied by changes in a variety of demographic and socioeconomic circumstances. Collectively, this work demonstrates that theory development and empirical testing can provide a solid conceptual foundation to improve our understanding of how food availability influences unhealthy diet and obesity, for different groups of people and across a range of circumstances. The application of this systematic approach could lead to a more nuanced view of mechanisms of action and thereby more effectively address complex public health problems.