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The pain response, an evolutionary adaptation which promotes tissue integrity and survival, is prevalent in patients with critical illness or injury. However, pain in the intensive care unit (ICU) can be maladaptive by increasing suffering and contributing to physiologic perturbations, agitation, ventilator dyssynchrony, and impaired mobility. The accurate assessment of pain and precise targeting of analgesic therapy is therefore a fundamental goal of intensive care medicine. Here, we review pain assessment tools which have been developed and/or validated for use in the ICU. Direct pain evaluation in critically ill patients is challenged by barriers to assessment such as intubation, sedation, brain dysfunction or injury, and, more recently, paralysis for management of severe ARDS due to COVID-19. In these instances, pain assessment is relegated to indirect measurement tools whose validity and reliability are not well established. Protocol-based pain surveys in the ICU have been associated with reductions in pain scores, opioid requirements, duration of mechanical ventilation, and ICU length of stay. Research aiming to test and validate specific and sensitive pain biomarkers could lay the foundation for more precise, effective, and safe treatment of pain in the ICU.