The assessment of pain is the foundation of pain treatment. Without the ability to measure pain, it is impossible to evaluate the impact of any intervention aimed at alleviating it. This is particularly true for vulnerable populations, such as infants (Latin – infans or voiceless), who cannot verbally report their pain to us and, therefore, require proxies for self-report of pain. Starting in the 1980s, committed cadres of investigators have attempted to develop psychometrically valid instruments that could allow us some insight into the infant's experience of pain. Initially, these tools relied on physiological variables (heart rate, respiratory rate, and blood pressure), then behavioral observations (facial expression, movement patterns, crying, and consolability) and, ultimately, some composite of the two. Although modifications were required for preterm infants and occasional studies suggested a lack of coherence between the various variables, there evolved a cluster of instruments that were universally accepted as having the ability to offer us at least a general indication of infant pain.