A valid pain assessment is the foundation of adequate pain management. Pain assessment can be challenging, especially in adult intensive care unit (ICU) patients who are unable to self-report. In such situations, relying on observational assessment tools is an alternative strategy. This review describes and analyzes the development and psychometric properties of pain assessment tools developed for use with nonverbal critically ill adults. A total of 32 relevant papers that described the psychometric properties of eight pain assessment tools were included. The scale development process, psychometric properties (i.e., reliability and validity), and feasibility of pain assessment tools were analyzed using a 0 to 20 scoring system. Each pain assessment tool was scored independently by two reviewers. Of the eight behavioral pain scales developed for use in adult ICU patients, the Behavioral Pain Scale (BPS) and the Critical-Care Pain Observation Tool (CPOT) are considered to be the most valid and reliable for this purpose, according to the available evidence. Behavioral pain scales may be viable alternatives to assessing pain in ICU patients who are unable to self-report, but only valid, reliable, and feasible scales should be used for this purpose.