As part of some treatments of depression, patients are asked to keep an activity diary on a sheet of paper in which they log their activities, and their emotions or mood. However, this way of keeping an activity diary has many disadvantages. A mobile app seems like a better solution. But how must an app for patients with depression be designed, to facilitate quick and effortless capture of activities, and emotions and/or mood? We used a human-centered development process with multiple design iterations and evaluations to develop an activity diary app for people with depression. In the process differing perspectives of therapists and patients became apparent, as well as within the group of therapists themselves. The resulting app was well-received by patients and therapists.