© RSNA, 2017. Transarterial chemoembolization (TACE) is considered a standard local-regional treatment for intermediate-stage hepatocellular carcinoma (HCC) and the most common bridging therapy. This treatment is offered to more than 70% of patients who are on the waiting list for liver transplantation in the United States. HCC typically receives its blood supply from the hepatic artery; however, it can recruit a parasitic supply from extrahepatic collateral (EHC) arteries. The development of an EHC arterial blood supply can interfere with the therapeutic efficacy of TACE and result in treatment failure and poor outcome. Cross-sectional imaging—specifically computed tomography and magnetic resonance imaging—has some limitations in depicting the presence or absence of an EHC arterial supply during the pre-TACE evaluation. Catheterization and angiography of every possible EHC artery during a routine TACE procedure would be time consuming and technically challenging and would not always be feasible. Therefore, the prediction of a potential EHC arterial supply on the basis of tumor location before, during, and after TACE is fundamental to achieving optimal therapeutic efficacy. To perform TACE through EHC arteries, special considerations are necessary to avoid potentially serious complications. The authors review the factors influencing the development of an EHC arterial blood supply to HCC and describe a systematic approach to enhance the ability to predict the presence of EHC arteries. They also describe the proper technique for TACE of each EHC artery and how to avoid potential technique-related complications.