Nurses play a critical role in the assessment and management of infant pain. In this article, infant pain assessment tools that are valid, reliable, clinically sensitive, and developmentally appropriate are discussed. Their purpose, use in acute pain, and applicability for premature and newborn infant populations are described. Behavioral and physiological indicators of pain, sleep-wake states, and infant development are discussed as important considerations in pain assessment. Recommendations for improving pain assessment practices are offered in conclusion.