Biological sex is a set of biological (genetic, hormonal, anatomical, morphological) characteristics of an organism, primarily determining the role of this organism in the reproductive process. The set of characteristics includes: Genetic sex (set of chromosomes); Gonadal sex (gonads); Hormonal sex (predominance of sex hormones in the blood); Internal genital sex (structure of the internal genital organs); External genital sex (structure of the external genital organs); Subsidal and sensory sex (development of secondary sexual characteristics). Was it worth talking about this topic if we could limit ourselves to the usual “school” description of XX women and XY men? Within the framework of this article, we talk only about biological sex, without involving the concept of gender. Genetic sex cannot always be understood unambiguously: there are many variations in the set of sex chromosomes, for example, XXYY, XYY, XXY and others. Genes on chromosomes influence phenotypic traits. Intrauterine development of the fetus occurs under the influence of many biochemical processes and depends on a large number of environmental factors, and due to individual development, various modifications may be present. All variations in the development of an organism other than “typically female” and “typically male” are called intersex variations. It is important to note that issues of biological sex are issues of physical and mental health from birth. This topic does not at first glance relate to psychology, however, behind the simple words “Intersex people make up 1.7% of the population” there are many gaps in scientific knowledge about the needs intersex people, the psychological consequences of binary gender socialization, the psychological consequences of surgical interventions in infancy and many others. One could talk for a long time about the variety of types of development of congenital differences in intersex people, about the fact that they are all normal and healthy options for human development, but this can be dealt with much better and described in more detail by the Association of Russian-speaking intersex people. My article is for informational purposes only..