Background: Type 2 Diabetes Mellitus (T2DM) represents a major public health concern. Over one million annual deaths worldwide are attributed to the disease, making it the ninth leading cause of mortality. In Ireland, the number of adults over 40 years of age with or at high risk of developing T2DM is estimated at 408,000 (17.4%) and could reach 717,000 by the year 2036. To reverse the escalating trend of T2DM, several countries have implemented a diabetes prevention programme (DPP) which empowers people at risk of T2DM to maintain a healthy weight and blood glucose levels through regular physical activity and heathy eating. Digital DPPs have also been developed to facilitate online participation via computer or smartphone. A national DPP is currently under consideration in Ireland; and, given the recent digital health boom, a digital programme could have significant impact. However, it is unknown which technology-driven T2DM prevention interventions are effective in producing clinically significant improvements in T2DM-related outcomes, and which intervention components have the greatest impact. Furthermore, the factors affecting the acceptability of a digital DPP among its target users remain unknown. Identifying these components and factors is essential for the development and implementation of an engaging and effective programme. Aim: This research aimed to assess the effectiveness of, and active ingredients in, digital T2DM prevention interventions. It also aimed to identify and explore factors that influence the acceptability of a digital DPP among adults living in Ireland, particularly those at risk of T2DM. The findings of this research will form an evidence base for the development and implementation of an Irish digital DPP. Methods: A mixed methods approach was adopted, informed by the UK Medical Research Council framework for developing and evaluating complex evaluations. The research comprises three studies. In study one, a systematic review was conducted to assess the effectiveness of technology-driven T2DM prevention interventions, and to identify the behaviour change techniques (BCTs) and digital features frequently used in effective interventions. In study two, a digital health acceptability model was developed and tested via cross-sectional questionnaire and structural equation modelling to identify the factors that influence the intention of adults living in Ireland to use a digital DPP. Study three used semi-structured interviews and qualitative content analysis to further refine the model developed in study two and to explore the views and experiences of adults at risk of T2DM regarding factors that affect the acceptability of a digital DPP, such as health status, social influence, health technology use, health behaviours, and perceptions of a smartphone based digital DPP. Findings: According to the systematic review, in the short term (≤6 months), 63% of technology-driven T2DM prevention interventions achieved a clinically significant weight loss of at least 3%. However, only 33% of interventions achieved the 5% weight loss benchmark at ≥12 months. Of the interventions that reported glycaemic status, 77% and 38% reported a significant improvement in HbA1c and fasting glucose respectively. Interventions containing a larger number of BCTs and digital features were more effective. The BCTs: social support (unspecified), goal setting (outcome/behaviour), feedback on behaviour, and self-monitoring of outcome(s) of behaviour were most effective. Interventions containing digital features that facilitated health and lifestyle education, behaviour/outcome tracking, and/or online health coaching were most effective. In study two, 316 eligible participants (Mage = 36) completed the questionnaire, of which 42% had a slightly elevated or higher risk of T2DM. The acceptability model developed in this study explained 65% of the variance in the intention to use a digital DPP. Twelve direct factor relationships were statistically significant. Subjective norm had a moderate-to-large impact on T2DM risk perceptions. Health status, perceived susceptibility to T2DM, eHealth readiness, communicative eHealth literacy and image had significant impacts on use intentions through mediators of perceived ease of use and perceived usefulness. In study three, 17 adults (Mage = 50 years) at risk of T2DM participated in a semi-structured interview. Descriptive themes relating to personal health, social influence, eHealth literacy, healthy eating, physical activity, and perceived usefulness plus ease of use of a digital DPP were identified. Health technologies, programme features, and interactions with friends and health professionals regarding their health behaviours were viewed by participants as both favourable and unfavourable, potentially affecting digital DPP acceptability. However, the desire for a programme to be tailored at both the individual (e.g., personalised goals) and group (e.g., homogenous peer groups) levels was a common thread. Conclusion: The findings of this research have advanced the evidence base regarding T2DM in Ireland, laying the foundation for the development and implementation of a national digital DPP. This research has also advanced the international knowledge base on what works in digital T2DM prevention interventions in three ways. First, it has extended the current understanding of health beliefs and eHealth literacy. Second, it identified the need for policies that improve access to healthy foods and food skills training. Third, it recommends several further research and practice avenues, including the improved measurement and reporting of key programme outcomes, and consultation with healthcare professionals to facilitate programme buy-in. These avenues are vital in understanding a digital DPP’s mechanisms of action, enhancing programme engagement and effectiveness, and ensuring significant and sustained impact on T2DM.