In this thesis I argue that negative attributions mediate between attachment insecurity and relationship outcomes. Using a sample of 59 couples the well-documented association between attachment insecurity and relationship satisfaction was replicated. I then tested whether this association was mediated by attributions for hypothetical behaviour for a real partner and a hypothetical potential partner. Attributions for real partner behaviour did mediate between insecure attachment and relationship satisfaction, but not attributions for a potential partner. It was further hypothesised that an association would exist between couple communication and attachment insecurity, which would be mediated by negative attributions. Couples completed two ten-minute problem-solving discussions and participated in a video-mediated recall process, providing a measure of attributions for real events with their current partner. Results supported the hypotheses for self-reported, but not observed, communication. It was concluded that the association between attachment and attributions does not represent a consistent cognitive processing bias, but rather a relationship-specific phenomena. Future research directions were proposed to investigate mediation from a long-term perspective and the therapeutic implications of these findings were discussed.