After an introduction dealing with the "historical evolution" of the Noonan syndrome (NS), we try to define the NS phenotype based on clinical descriptions published since 1883. The theories concerning the cause of the NS are discussed fully. The peculiar cardiac involvement deserves special attention and raises the question of whether the Watson and LEOPARD syndromes are indistinguishable from NS. Finally, the recent contributions to the variability of the NS phenotype (reports on lymphatic dysplasia, partial deficiency of factor XI, malignant hyperthermia, perceptual-motor disabilities, and endocrine evaluation) are also described.