The anatomy of cancer patients changes between radiation treatment planning and delivery as well as over the course of radiotherapy. Adaptive radiotherapy (ART) aims to deliver radiation accurately and precisely in the presence of such changes. To that end, ART uses an imaging feedback loop to quantify these changes and modify the treatment plan accordingly. This paper provides an overview of anatomical changes occurring over the course of therapy and various adaptive strategies developed to account for those. Moreover, residual uncertainties present in adaptive radiotherapy are discussed as well as required tools, potential pitfalls and remaining challenges.