Transactional theories of stress and coping (e.g., Lazarus & Folkman, 1984, 1987) outline how individuals appraise the significance of stressful events in relation to their goals and the context; with appraisals proximally linked to ways of coping with stress and, in turn, wellbeing outcomes. Yet, appraisals and subsequent coping responses are known to show individual differences and these differences have been found, in some literatures, to be influenced by metacognitive processes, sometimes referred to as decentering. Decentering has been defined as the ability to shift perspective and step outside of one's own mental events (i.e., conscious cognitive function). As illustrated in this definition; decentering involves the capacity to reflect on, critique, evaluate, and refine personal judgements, and these metacognitive processes can shape individual differences within individuals' stress appraisals and coping deployment across the stress-coping-adjustment process. Accordingly, the main purpose of this thesis was to test whether decentering metacognitive processes are uniquely associated with stress responding related to couple (romantic relationship) stressors and relationship wellbeing through the conduction of three empirical studies. After focusing on the measurement of decentering in Study 1, Studies 2 and 3 concentrated on stress within romantic relationships and drew upon stress-coping theory and the decentering metacognitive processes model to address how decentering and ways of coping with romantic relationship stress interrelate and uniquely relate to relationship wellbeing. Across the studies, multiple methods were used, including surveys and observation, collecting data from individuals (Studies 1 and 2) and couples (Study 3). [...]