Considerable progress has been made in achieving a perspective of the pathophysiology of ciliary defects in human disease in the interval between Siewert's and Kartagener's early descriptions of KS. Not only have we achieved a better understanding of some of the mechanisms involved in the pathogenesis of congenital ciliary syndromes, but also we have come to appreciate a new spectrum of ciliary defects, those of acquired etiology. The advent of modern electron microscopy has been a significant element to this progress and its importance is reflected by the numerous studies of ciliary defects that have come in the wake of the pioneering ultrastructural investigations of Afzelius and colleagues. However, each discovery appears to generate additional questions. This is indicative of the importance of the subject to both basic and clinical science and of the vitality of the investigators in this field as a comprehensive understanding of ciliary diseases is sought.