The Assessment of Preterm Infants' Behavior (APIB) is a newborn neurobehavioral assessment appropriate for preterm, at risk, and full-term newborns, from birth to 1 month after expected due date. The APIB is based in ethological--evolutionary thought and focuses on the assessment of mutually interacting behavioral subsystems in simultaneous interaction with the environment. The subsystems of functioning assessed include the autonomic (respiration, digestion, color), motor (tone, movement, postures), state organization (range, robustness, transition patterns), attention (robustness, transitions), and self-regulation (effort, success) systems as well as the degree of facilitation required to support reorganization and subsystem balance. The environment is represented by a sequence of distal, proximal, tactile, and vestibular challenges, derived from the BNBAS. The APIB conceptualizes infant competence as the degree of differentiation of subsystem function and degree of modulation of subsystem balance at any stage in infant development. Infants are understood as actively seeking their next differentiation, while counting on good enough environments to assure progressing developmental competence. In the case of interference such as premature birth, the mismatch of expectation and actual experience causes misalignment, which may become developmentally costly. The assessment is a finely tuned dialogue between examiner and infant, which requires training, skill and self-knowledge. The APIB has well established inter-rater-reliability, concurrent and construct validity, and is clinically relevant for behavioral intervention and individually appropriate and supportive care. (Contains 3 figures.)