Pain assessment poses a great challenge for clinicians in intensive care units. This descriptive study aimed to find the most reliable, sensitive, and valid tool for assessing pain. The researcher and a nurse simultaneously assessed 47 nonverbal patients receiving mechanical ventilation in the intensive care unit by using 3 tools: the Behavioral Pain Scale (BPS), the Critical-Care Pain Observation Tool (CPOT), and the adult Nonverbal Pain Scale (NVPS) before, during, and after turning and suctioning. All tools were found to be reliable and valid (Cronbach α = 0.95 for both the BPS and the CPOT, α = 0.86 for the NVPS), and all subscales of both the BPS and CPOT were highly sensitive for assessing pain ( P < .001). The NVPS physiology ( P = .21) and respiratory ( P = .16) subscales were not sensitive for assessing pain. The BPS was the most reliable, valid, and sensitive tool, with the CPOT considered an appropriate alternative tool for assessing pain. The NVPS is not recommended because of its inconsistent psychometric properties.