The article discusses the modified RECIST (mRECIST) assessment criteria for evaluating response in hepatocellular carcinoma (HCC). The traditional World Health Organization (WHO) and Response Evaluation Criteria in Solid Tumors (RECIST) criteria have limitations, particularly when applied to molecular-targeted therapies or locoregional therapies. In 2000, the European Association for the Study of the Liver (EASL) amended RECIST to include tumor necrosis, which was later endorsed by the American Association for the Study of Liver Diseases (AASLD). The AASLD-JNCI guidelines introduced the mRECIST criteria, which focus on viable tumor tissue showing uptake in the arterial phase of contrast-enhanced imaging. These criteria aim to provide a more accurate assessment of treatment response in HCC. The article outlines the background on response assessment, the need for guidelines in HCC research, and the design of clinical trials. It also details the mRECIST criteria for assessing target lesions, nontarget lesions, and new lesions, emphasizing the importance of standardized imaging techniques and centralized radiologic review. The mRECIST criteria are expected to improve the reliability of response assessment in HCC clinical trials and align with hard endpoints such as survival.The article discusses the modified RECIST (mRECIST) assessment criteria for evaluating response in hepatocellular carcinoma (HCC). The traditional World Health Organization (WHO) and Response Evaluation Criteria in Solid Tumors (RECIST) criteria have limitations, particularly when applied to molecular-targeted therapies or locoregional therapies. In 2000, the European Association for the Study of the Liver (EASL) amended RECIST to include tumor necrosis, which was later endorsed by the American Association for the Study of Liver Diseases (AASLD). The AASLD-JNCI guidelines introduced the mRECIST criteria, which focus on viable tumor tissue showing uptake in the arterial phase of contrast-enhanced imaging. These criteria aim to provide a more accurate assessment of treatment response in HCC. The article outlines the background on response assessment, the need for guidelines in HCC research, and the design of clinical trials. It also details the mRECIST criteria for assessing target lesions, nontarget lesions, and new lesions, emphasizing the importance of standardized imaging techniques and centralized radiologic review. The mRECIST criteria are expected to improve the reliability of response assessment in HCC clinical trials and align with hard endpoints such as survival.