Obesity is considered a significant risk factor for negative physical health and psychological outcomes. However, factors which account for some of the observed relationship have been under investigation for some time, and as a result have challenged the idea that obesity itself is the cause of the physical health and psychological outcomes. The picture is known to be complex, with influence of biological, psychological, social and behavioural factors. This thesis aimed to increase understanding of the factors involved in the relationship between obesity and psychological distress. In particular, to assess the role of physical self-concept and social comparisons in this relationship. Part one of this portfolio presents a systematic literature review of the relationship between physical self-concept and psychological wellbeing. The findings of the review suggest that positive physical self-concept is associated with better psychological wellbeing, and it highlighted the importance of physical self-worth and global self-esteem on mental health outcomes. Part two presents an empirical paper that investigated the mediating role of physical self-concept in the relationship between Body Mass Index (BMI) and psychological distress in the general population. The findings suggested that physical self-concept mediates the relationship between BMI and psychological distress, and once this indirect effect is accounted for, the relationship between BMI and psychological distress changes in strength and valence. In addition, the study suggests the important influence of social comparisons and social norms in this model.