Timing and sequencing of acquisition of developmental capabilities are important issues in view of the growing interest in early developmental evaluations and interventions. Background factors are (1) continuity versus discontinuity of development; (2) the extensive variability of normal individual development; (3) prognostic significance of suboptimal test performance; (4) alternative pathways of mental development demonstrated by physically handicapped children. This last point is explored in detail by means of data from a study of children with defects due to congenital rubella.