Assessment and management of patients' pain across practice settings have recently received the increased attention of providers, patients, patients' families, and regulatory agencies. Scientific advances in understanding pain mechanisms, multidimensional methods of pain assessment, and analgesic pharmacology have aided in the improvement of pain management practices. However, pain assessment and management for critical care patients, especially those with communication barriers, continue to present challenges to clinicians and researchers. The state of nursing science of pain in critically ill patients, including development and testing of pain assessment methods and clinical trials of pharmacological interventions, is described. Special emphasis is placed on results from the Thunder Project II, a major multisite investigation of procedural pain.