In Australia, up to 53 women will be diagnosed with breast cancer every day of 2019. With improved screening, diagnosis and treatment, survival rates are over 90% for 5 years after diagnosis. Notably, even when individuals are cancer-free following treatment for breast cancer, it is considered a chronic illness with recurrence a constant possibility. Many individuals adapt to this uncertainty; however, some are at risk of ongoing psychosocial distress. For individuals diagnosed with cancer, from the time of diagnosis, throughout treatment and into survivorship, maintenance of wellbeing is critical. Mindfulness and self-compassion are two positive constructs known to protect against distress, particularly during times of adversity. This research had three aims: 1) to identify the unique and common contributions made by dispositional facets of mindfulness and positive and negative self-compassion, to protect against anxiety and depression and promote quality of life in a sample of individuals with breast cancer; 2) to determine the stability and reliability of dispositional mindfulness and self-compassion over time; and 3) to evaluate the feasibility and efficacy of a facilitator-supported online mindfulness-based stress reduction (eMBSR) programme for women with breast cancer, many of whom were reporting significant psychological distress. To address these aims, three studies were conducted to explore facets of mindfulness and self-compassion that might protect against distress and promote wellbeing for people with breast cancer, incorporating cross-sectional, longitudinal, and interventional methods to examine these associations. In Study 1 the associations between the dispositional facets of mindfulness, positive and negative self-compassion, and reports of anxiety, depression, and quality of life were investigated in a sample (N = 199) of individuals diagnosed with breast cancer who were in or had completed treatment for breast cancer within the past 36 months. Whilst there was considerable overlap between these constructs, regression analysis found that the mindfulness facets, act with awareness and being non-judgemental of inner experience, together with positive self-compassion, were uniquely associated with lower reported symptoms of depression and anxiety and better quality of life. Higher reports of negative self-compassion were associated with higher reports of anxiety and depression and symptoms of poorer quality of life. Study 2 examined the influence of dispositional mindfulness and positive and negative self-compassion on ongoing levels of anxiety, depression, and quality of life over time. Participants (N = 79) completed two additional questionnaires, three (Time 2) and six months (Time 3) following participation in Study 1. The associations between the facets of mindfulness and outcome variables were also stable over time. When examining trends over time, anxiety, depression, all facets of mindfulness, and positive and negative self-compassion were stable, showing evidence of high reliability. Time 3 quality of life significantly improved from Time 1 and 2. Increased mindful non-judgement and decreased negative self-compassion consistently predicted lower levels of anxiety and depression. Increased mindful non-judgement and lower negative self-compassion, and to a lesser degree increased reports of acting with awareness and positive self-compassion, were also associated with improved quality of life over time. When examined together, Studies 1 and 2 indicated that being more mindful, particularly less judgemental of the self, as well as identifying less with difficulties and being less self-critical, were associated with reduced anxiety and depression and better wellbeing in a distressed sample. Study 3 evaluated the feasibility and efficacy of an eMBSR programme for women with breast cancer, most of whom reported significant psychological distress (81.3%). All 16 participants were recruited from Study 1 and split across two parallel eMBSR programme groups using quasi-random allocation. A single case multiple baseline design was used. In addition, a normative control (N = 79) sample was taken from Study 2 for comparison. Feasibility results showed an acceptable intervention completion rate (62.7%) and good participant engagement with the programme. Results of the study were very positive and compelling. Analysis of case series data suggested participants reported positive personal benefits from programme participation. Further, analysis of pre-, post-, and follow-up intervention data found significant improvements in anxiety, mindful non-reactivity and observe, and positive self-compassion. No significant changes were found for depression, the other mindfulness facets, or negative self-compassion. This study provides promising results for further investigation of eMBSR delivery format. Overall, the findings provide insights into how being more mindful, and showing more positive and less negative compassion toward the self, promotes wellbeing in individuals diagnosed with breast cancer. Dispositional mindfulness and self-compassion were stable over time in the absence of intervention, indicating that training to enhance these skills might be helpful in improving wellbeing over time. The mindfulness facet of non-judgement and negative self-compassion were found to be the strongest and most consistent unique predictors of anxiety, depression, and quality of life. Mindful acting with awareness and positive self-compassion were also found to predict anxiety, depression, and quality of life. These findings could be used to inform creation of more targeted interventions. Finally, the results for eMBSR provide very promising and compelling initial feasibility and preliminary efficacy data that show patterns of improvement consistent with past face-to-face MBSR research. Several avenues for further research are suggested, including more representative samples and improvements to the intervention interface by using updated software solutions. This thesis represents a significant contribution to the understanding of how mindfulness and self-compassion can protect against distress and promote wellbeing during times of adversity work, and how they can be influenced using a novel internet version of an evidence-based programme.