Vestibular-related dizziness is a highly debilitating condition, especially common amongst older adults. It can often be successfully managed with physical therapy exercises known as Vestibular Rehabilitation (VR) but access to VR is very limited. The 'Balance Retraining' digital intervention was developed to meet the need for increased availability of this therapy, and a primary-care based Randomised Controlled Trial demonstrated it to be effective. In order to maximise the potential efficacy, reach and acceptability of such interventions, it is important to investigate causal mechanisms and contextual factors. This provides insight how and why outcomes occur under given circumstances, and the underlying psychological mechanisms. This thesis employed quantitative and qualitative methodologies to investigate the processes involved in individuals' engagement with, and outcomes of, the Balance Retraining intervention. A systematic review of 30 studies investigating predictors of adherence to self-managed physical therapies revealed that self-efficacy, self-motivation, intention, previous adherence behaviour and social support were most consistently associated with adherence. Following this, a qualitative study explored older adults' experiences of engaging with Balance Retraining over a 6-week period. Participants experienced internet-delivered VR very positively; many were motivated by improvements in their symptoms, but some also found that practical issues, or doubts about the exercises caused difficulties. A quantitative investigation of predictors of change in dizziness severity revealed that adherence-problems, particularly those relating to symptom exacerbation, were especially detrimental for dizziness outcomes and that those who were more anxious or less confident about their ability to complete VR were more susceptible to these. A final mixed methods analysis found that the intervention appeared to operate primarily through addressing adherence problems and that greater overall use, and use of specific features, was beneficial for outcomes. The findings of this thesis provide promising evidence that digitally-delivered VR is acceptable and accessible amongst older adults, and highlight key intervention features and the underlying BCTs that appear instrumental in Balance Retraining's effectiveness.