Pain assessment is an essential foundation to mitigate pain and its consequences in the developing child. However, pain assessment in neonates and infants is challenging and, to date, there is no “gold standard” infant pain indicator, measure, or approach. This chapter encompasses (1) a comprehensive evaluation of the most current and well validated neonatal/infant pain assessment measures; (2) an overview on biomarkers and cortical indicators on neonatal/infant pain; (3) the integration of recommendations on pain-assessment measures and practices within clinical practice guidelines, policies, and procedures; and (4) challenges associated with neonatal and infant pain assessment in terms of research, clinical, and knowledge translation issues.