A brief history of the development of modern technics in human cytogenetics is presented, and application of these technics to the study of human disease is illustrated. The basic chromosomal defects in several disease states are described. The best studied conditions to date are mongolism, Klinefelter's syndrome and Turner's syndrome. In mongolism there is trisomy for the twenty-first autosome. In Klinefelter's syndrome there is an XXY sex chromosome constitution. In Turner's syndrome there is an XO sex chromosome constitution. The chromosome numbers in these conditions are, respectively, forty-seven, forty-seven and forty-five, whereas the normal human chromosome number is forty-six. Non-disjunction of chromosomes at meiosis, in the production of parental gametes, is postulated to be the mechanism for the production of the chromosomal aberrations in the three conditions mentioned. The process of non-disjunction is discussed in detail. It results in the production of parental gametes with either more or less than the normal haploid number of chromosomes. Fertilization involving such a gamete and another normal one may result in a zygote that is either trisomic for the chromosome involved or is deficient in one chromosome. Other types of chromosomal aberrations are considered. These include translocation, mosaicism, and isochromosome formation. The bearing of recent information on the theory of sex determination is also discussed.