Imaging plays a central role in the follow-up of patients under systemic treatment for advanced or metastatic cancers, both in clinical trials and in routine. Indeed, measurements of tumoral lesions have always been used to monitor treatment efficacy: Before imaging, clinical examination was requested, and the reliability and reproducibility of tumor evaluation were already questioned. In 1981, the World Health Organization (WHO) published recommendations for “reporting results in cancer treatment”: The goal of this publication was “to make it possible for investigators to compare validly their results with those of others.” These first recommendations paved the way to tumor evaluation with imaging. Nowadays, its use in cancer patient monitoring is mandatory in clinical trials with the adoption of Response Evaluation Criteria in Solid Tumors (RECIST) that were proposed by an international academic working group and largely accepted in the oncologic community. Nevertheless, due to the rapidly evolving landscape of anticancer therapies targeting different tumor components, RECIST criteria are sometimes insufficient for treatment assessment, and other criteria can be used. This chapter aims at describing the principles of tumor evaluation on imaging.