N of 1 trials are single-subject, randomised, crossover studies, in which the patient serves as their own control to compare the efficacy of a treatment. They offer an alternative to large, randomised clinical trials, which have limited applicability, and empirical testing, which is arbitrary in nature. N of 1 trials are regarded as providing the highest strength of evidence for individual subject decisions. They rely upon the patient expressing preferences and making shared decisions for determining the future therapy, and may therefore lead to the successful implementation of lifestyle interventions. N of 1 trials are an optimal approach for making therapeutic decisions in chronic diseases like diabetes mellitus, where decision-making is often reliant upon arbitrary criteria and clinical judgement.