OBJECTIVE. CT remains the imaging modality of choice in the diagnosis of colorectal cancer (CRC) and anal cancer. However, advances in imaging have expanded the role of MRI and PET/CT. This article focuses on the evolving role of FDG PET/CT in the diagnosis, radiation therapy planning, therapy assessment, and posttherapy monitoring of CRC and anal cancer. CONCLUSION. FDG PET/CT is a valuable imaging modality that impacts the clinical management of patients with CRC and those with anal cancer.