Using data from 179 preterm infants, a neurobehavioral maturity assessment was developed by using a process in which clusters characterized by conceptual coherence and face validity were systematically subjected to statistical analyses designed to test whether they also had high test-retest reliability, statistical cohesion, and developmental validity. The psychometric soundness of the test items was made a precondition for their inclusion into the assessment procedure. Also tested were cluster redundancy, as well as the impact of gestational and conceptional age, and of postbirth influences on the functions tested. 8 dimensions of neurobehavioral functioning were found to be stable with a test-retest reliability of at least .6 or higher on 2 consecutive days, nonredundant and developmentally valid. They were: Active Tone/Motor Vigor, Alertness and Orientation, Excitation Proneness, Inhibition Proneness, Scarf Sign, Popliteal Angle, Maturity of Vestibular Response, and Vigor of Crying.