This paper examines curriculum differentiation (i.e. streaming or ability-grouping). After placing curriculum differentiation in an international perspective, it outlines the main conclusions from empirical research on differentiation over several decades. Against this empirical background, it describes and considers the three specific orientations towards curriculum differentiation: a genetic perspective, a cultural perspective, and a sociological perspective. The insights from the various perspectives are integrated and expanded in a framework of curriculum theory, research, and practice.