National Health Service (NHS) policy suggests that increasing usage of electronic personal health records (PHR) by patients will result in cost savings and improved public health, especially for people with long-term conditions. PHR design features are inevitably important, since a good PHR design should make the users achieve their health goals effortlessly, which is understandable and usable. Three original theoretical models were developed using realist evaluation, one per long-term condition cohort, describing the interaction between the PHR design features and the patient and disease specific factors, to help determine what works for whom in what circumstances.