Feedback from digital technology has often been used to support people in changing undesired, unhealthy habits. As yet, there has been little research into the efficacy of these designs. In my PhD project, I evaluated the acceptance, sustained use, and effect of four designs that provide feedback on undesired habitual behaviour through digital technology. Findings are that the disruptive effect of feedback on undesired habits has been proven, and there is some evidence that feedback may have a lasting effect on behavioural change. (Sustained) use of digital designs that provide feedback is moderated by motivation, age, goal-related aspects, and user experience. The necessity of high motivation to use a device poses challenges for the acceptance of and sustained engagement with designs for behaviour change that rely on feedback. Further challenges concern privacy and the quality of the evaluations of our designs.