Disorders of sex chromosome, X and Y, consist of abnormality of the number or structure of the sex chromosome. Because sex chromosomes have a variety of genes related to sexual differentiation, disorders of sex chromosome induce a variety of disorders of sexual differentiation. At first, in this title, the process of normal sexual differentiation is shown. Classical disorders of sex chromosome are Klinefelter syndrome, XX male, XYY male, Turner syndrome, XXX female, and XY female. True hermphroiditism, mixed gonadal dysgenesis, and pure gonadal dysgenesis are also included, because most of these disorders have abnormal sex chromosome. Molecular analysis of sex chromosome is clarifying disorders with a minute abnormality of sex chromosome. They include male infertility, premature ovarian failure, and fragile X syndrome. Explanations of the above disorders are given briefly.