Fatigue is a common and distressing long-term consequence of cancer. Chronic cancer-related fatigue affects work ability, hampers in maintaining social relations, and impacts patients’ well-being. Most treatments for chronic cancer-related fatigue focus to some extend on changing physical behavior, but there is a large variability in the way they do this. This thesis focuses on an online intervention that targets physical behavior by using an ambulant activity coach. In this ambulant activity feedback therapy (AAF), a physiotherapist guides patients following a nine week intervention protocol. The ambulant activity coach, consisting of a hip worn accelerometer and a smartphone, provides feedback on a patient’s physical behavior during the day and reinforces a physical activity goal that the patient defines together with the physiotherapist. A three-armed randomized controlled trial was performed to test AAF and an online mindfulness-based cognitive therapy next to a control group that received weekly psycho-educational information. This trial has led to a number of studies, and provided insight in the heterogeneity of the physical behaviors of the study population, in the effectiveness of the intervention in terms of fatigue, mental health, and perceived work ability, and in its working mechanisms. As two thirds of the participants in the AAF group reduced fatigue clinically significantly, and effects remained up to a year after randomization, it was concluded that AAF is a promising intervention for reducing chronic cancer-related fatigue.