For over two decades, researchers have studied the expression of pain in young infants to unlock the nature of this powerful experience early in life, with these studies resulting in the discrediting of numerous myths about infant pain (e.g., infants are insensitive to pain). The more useful measures of infant pain to emerge from this research examine facial activity, body movement and cry characteristics. To date, however, there has been little effort to examine the developmental progression of these pain behaviours throughout early infancy. Using the sociocommunications model of pain as a theoretical framework, this work examines pain expression in infants receiving routine immunization injections and their parents. This book describes how pain expression differs throughout the first year of life, illustrates how parent perceptions and assessment of pain change with the development of the infant and explores the relationship between parental assessments and behavioural indices of infant pain. This work has great relevance for caregivers of pre-verbal infants who often are asked to assess the presence or absence and severity of pain an infant may be experiencing.