PRO-MENTAL is a non-interventional, prospective, observational study investigating longitudinal associations between diabetes distress, mental disorders, and glycemic outcomes in people with type 1 diabetes (T1D) and type 2 diabetes (T2D). The study aims to determine mental health subtypes, trajectories, and patterns and to advance a precision medicine approach to improve mental health in people with diabetes through personalized care and interventions. A total of 1500 people with T1D or T2D will participate in the study, running over a 24-month period. Participants will be recruited at different levels of diabetes care including specialized centers and hospitals. The assessment includes a baseline assessment (clinical interview, questionnaire survey, and laboratory assessment) and four subsequent measurement time points - every six months - to a total period of two years. Each measurement time point includes an online questionnaire survey as well as a 14-day ambulatory assessment of daily mental and somatic variables (smartphone-based ecological momentary assessment (EMA) of daily sleep quality, mood, stress, and diabetes-related burdens/distress, as well as continuous glucose measurement (CGM) of daily glucose levels). The study uses precision monitoring to identify evidence-based subgroups of people with diabetes with regard to mental disorders/problems and glycemic outcome. Epidemiological data regarding prevalence and incidence rates of depression, anxiety, and eating disorders will be analyzed, and patient trajectories and patterns will be determined. The study also aims to shed more light on the mediating mechanisms between mental health and glycemic outcomes. The findings of the study will be used as the basis to develop a precision medicine approach with personalized interventions for specific sub-groups of people with type 1 and type 2 diabetes.