There is no complete and satisfactory system of classification for constitutional bone diseases available at present. A combination of precise phenotypical description and genetical analysis has been shown, however, to increase our understanding of the basic defects and to allow an etiological classification of an increasing number of bone diseases. Examples are given to illustrate the usefulness of Mendelian analysis (autosomal dominant and recessive inheritance, X-linkage), microcytogenetics, cellular pathology, enzyme biochemistry, analysis of the gene products, and DNA analysis. The phenotype cannot be understood without knowledge of its genetic basis, but knowledge of the genetic basis laid down in the DNA is only useful in connection with knowledge of the phenotype, which cannot be derived from knowledge of the DNA.