Abstract In this chapter we consider gonadal maldevelopment caused by X-chromosome aberrations. As in Chapter 5, “gonadal maldevelopment” is defined broadly so that it includes defective spermatogenesis due to an extra X chromosome in XXY males and gonadal dysgenesis due to accelerated oocyte attrition in XO females. These two conditions, and XX/XY mosaics, are discussed under the heading, “Genomic Mutations”-an abnormal number of whole X chromosomes. This is followed by a section on gonadal maldevelopment due to abnormality in parts of an X chromosome, under the heading “Chromosomal Mutations.” Next we discuss the consequences of X chromosomes that bear a SRY-containing portion of the Y chromosome. Finally, we treat a prototypic example of gonadal maldevelopment due to mutation of a single X-linked gene.