This paper presents the clinical/developmental framework underlying a new diagnostic observational tool, the Disruptive Behavior Diagnostic Observation Schedule (DB-DOS). The special importance of observation for clinical assessment during the preschool period is delineated. The developmental rationale for a multi-dimensional assessment of disruptive behavior in young children, including problems in modulation of negative affect and low competence is discussed. The ways in which the DB-DOS will elucidate distinctions between normative and atypical behavior during this developmental period via (a) the integration of qualitative and quantitative dimensions of behavior within a clinically-sensitive coding system, (b) the observation of child behavior both within, and outside of, the parent-child context and (c) the use of specially designed tasks to "press" for clinically salient behaviors are addressed. The promise of this new method for yielding a more precise, developmentally based description of the phenotype of early onset disruptive behavior problems and for providing a standardized clinical tool for observational assessment of disruptive behavior in young children is presented. Large-scale validation of the measure is currently underway.