The Assessment of Preterm Infants’ Behavior (APIB) was developed by Heidelise Als in the late 1970s as part of the Synactive Theory of Development, which conceptualizes the preterm infant as an active participant in regulating physiological and behavioral subsystems. The APIB was designed to provide a structured, individualized assessment of neurobehavioral functioning in preterm and full-term infants, particularly in neonatal intensive care settings. Since its introduction, the APIB has been widely applied in both clinical practice and research to evaluate autonomic, motor, state, attention–interaction, and self-regulatory systems. Beyond its initial validation, the APIB has been used in at least 30 studies involving preterm infants assessed during the first four weeks after birth. These studies have yielded important insights into early neurodevelopmental functioning in this vulnerable population. Research using the APIB has provided critical information on respiratory stability, including patterns of breathing and autonomic regulation, as well as levels of motor activity and state organization. Importantly, the assessment has demonstrated its value in identifying infants’ emerging capacities for emotional regulation and self-soothing, highlighting early differences in stress responsivity and adaptive behaviors. Findings from these studies have informed individualized developmental care approaches, supporting interventions aimed at enhancing regulation, reducing stress, and promoting optimal neurodevelopmental outcomes in preterm infants.