Research shows a large variability in the degree to which adolescents with diabetes adjust to their illness. Adjustment to diabetes is important because it affects the mental health of adolescents, and has been shown to predict both adherence to treatment, and glycaemic control. This thesis proposed that attachment security has the potential to explain some of the variability in adjustment to diabetes. A causal pathway was hypothesised whereby attachment security both directly predicts adjustment to diabetes, and indirectly via choice of coping strategies. In addition, it was hypothesised that attachment security would indirectly predict adherence behaviours and glycaemic control. The relative importance of attachment to parents and peers was compared. The research hypotheses were tested in a sample of 99 adolescents aged 13-18 years, who had been diagnosed with diabetes for at least a year. Measures of attachment security, adjustment to diabetes, coping strategies, treatment adherence and glycaemic control were taken at one time point. The data were analysed using structural equation modelling to test the hypothesised causal pathway. The results suggested that attachment security both directly predicts adjustment to diabetes, and indirectly predicts adjustment to diabetes via avoidance focused coping strategies. Attachment to parents, but not to peers was associated with adjustment to diabetes, and some gender differences were observed. The results also suggested that attachment security indirectly predicts glycaemic control via adjustment to diabetes. There were no significant associations between adherence to treatment and any other variables. Neither attachment security nor adjustment to diabetes were associated with approach focused coping. It is concluded that attachment security may play an important part in both psychological and physical outcomes for adolescents with diabetes, and this has implications for attachment based interventions. The findings are discussed in relation to other theoretical models, and indications for future research are suggested.