Purpose of review: This review aims to report the latest discoveries regarding the relationship between BMI, obesity, and cancer development and treatment. Recent findings: Obesity and metabolic syndrome relationships with cancer have been deeply investigated in the literature but their association is still debated. Currently, it has been recorded an association between BMI and endometrial, colorectal, gastric, liver, bladder, and prostate cancer. The mechanisms behind this association have also been investigated. It has been hypothesized that chronic inflammation determined by obesity may concur to the development of tumors and that Insulin Resistance may enhance cell proliferation directly or indirectly. Moreover, different studies suggest that the relationship between higher BMI and cancer may include metabolic disturbances comparable to those linked to metabolic syndrome. However, greater weight has been linked to a better overall prognosis in patients with advanced disease, a concept called the obesity paradox. This paradox has been recently investigated in the context of urological malignancies, such as bladder, prostate, and kidney cancer. Summary: Patients’ metabolic and morphological status may impact their risk of developing different types of tumors and the response to systemic therapy. However, further research is necessary to better delineate the mechanisms behind these associations and how they could or should affect medical decision.