Background: The ongoing acquisition of genetic alterations in individual cancer cells leads to the emergence of genetically distinct tumor subclones. In concert with selective forces imposed on cancer cells by environmental factors, this drives tumor evolution. The stochastic nature of genetic events and the cellular adaption in response to external signals induces the formation of biologicallyheterogeneous tumor subclones. This intra-tumor cell heterogeneity (ITH) underlies the intrinsic (primary) and acquired (secondary) resistance to therapy that is commonly observed in patients with cancer and represents a major challenge for cancer (immuno-) therapy. Here we review the multiple dimensions of ITH, its impact on therapy resistance and existing approaches to overcome this still underappreciated therapeutic challenge. Methods: A selective literature review was performed using PubMed (keywords: “intratumor heterogeneity”; “tumor heterogeneity”). Results: ITH is a multilevel phenomenon that far exceeds genetic variation between tumor subclones alone. The tumor cells’ macro- and microenvironment contribute to epigenetic and metabolic alterations and induce phenotypic diversity between cancer cells of single lesions and of different tissue sites. Importantly, ITH and anti-tumor immune responses mutually influence each other. The immune system reduces ITH by eliminating immunogenic tumor subclones, while ITH, on the other hand, blunts spontaneous anti-tumor immune responses. ITH is associated with poorer responses to immunotherapy and poorer overall survival, highlighting the need for new therapeutic approaches to improve patient outcomes. While there are promising strategies aimed at reducing ITH, these have yet to prove their usefulness for the treatment of tumors with pronounced ITH. Conclusions: ITH is increasingly recognized as a major therapeutic challenge in patients with cancer. A better understanding and improved assessment of ITH will enable the prediction of therapy resistance and guide the development of new, more effective treatments.