1. 1. The biologic principle of bilateral symmetry is never manifested with mathematical precision, and even in fully homologous organs in the two halves of the body there are almost invariably small differences. 2. 2. Fundamental unilaterally directed forms of asymmetry are found, especially with respect to the internal organs of the alimentary canal and the circulation. 3. 3. Variable asymmetry can be classed as (a) qualitative, of the all-or-none type (for instance, the absence of teeth or the presence of supernumerary teeth) —and (b) quantitative, with a gradual transition from the smallest to the greatest degree of asymmetry that the organ in question can present. In both of these types one can distinguish between normal variants and anomalies. Varying asymmetry can either present, to an equal degree, left and right dominance or show a certain predominance on one side or the other. 4. 4. From the orthodontic standpoint, the following forms of asymmetry are of particular importance: 4.1. A. Qualitative asymmetry 4.1.1. (1) Number of teeth 4.1.2. (2) Cleft palate 4.2. B. Quantitative asymmetry 4.2.1. (1) Size of the teeth 4.2.2. (2) Location of the teeth in the arches 4.2.3. (3) Location of the arches in the head 5. 5. Pronounced anomalies of an asymmetrical nature are found in the head, the most marked being facial hemiatrophy and hemihypertrophy. 6. 6. Asymmetry can be of genetic or nongenetic origin. Among the former type are the fundamental forms for which the signals present in the inherited genetic constitution produce definite right-left differences. There are also, however, forms of asymmetry of a variable nature whose distribution is probably determined, to some degree at least, by genetic forces. A special type of such an asymmetry is the probably rare somatic mutation in which there is no inheritance of the trait by the following generations. The nongenetic forms of asymmetry can be determined by the influence exerted by the external environment, or they may be due to random developmental differences in the internal environment of the two halves of the body. In the case of some forms of asymmetry, the appearance of right-left differences is probably due to a combination of genetic and nongenetic influences; that is, the responsible genes display incomplete penetrance or irregular expressivity.