Ontogenesis of EEG sleep spindles is described and an ontogenic model presented. Spindle-development occupies approximately two weeks from onset of rudimentary spindles to development of mature spindles. In term infants rudimentary spindles are first clearly evident at postconceptional age 4 weeks; spindles are fully developed by age 11 weeks. Spindles of premature infants show a similar but relatively accelerated ontogenic course.