This article reviews the evaluation of pain in critically ill patients of all ages. Sources of pain and its physiologic significance are discussed. Factors influencing pain, perception, and behavior are also addressed to broaden health care professionals' vision of the difficulties associated with accurate pain assessment. The evaluation process, including a discussion of several tools ranging from visual analog scales to bispectral analysis, as well as goals of therapy are discussed. Finally, a practical approach to the assessment process, which supports the need for established, regularly used evaluation and treatment protocols, is proposed.