A main criterion to identify activity in phase II studies is the response rates achieved in a well-defined subset of patients. The response could be defined as a measure of tumor shrinkage. For 30 years, metric methods have been used to assess this response. The World Health Organization was the first organization to propose a unified definition for response status. Over time, the latter evolved and 10 years ago an international consensus panel proposed the Response Evaluation Criteria in Solid Tumors criteria. Although these guidelines for response assessment have limitations and biases, they have nevertheless been proven useful and advantageous. This article reviews those criteria and describes their use.