Nowadays, different sexualities and gender identities are quickly becoming more accepted in mainstream society. Despite this change, there are many people who believe that having a different sexual orientation or gender identity is a frowned upon choice. Research and brain biology are needed to disprove this belief. Brain research based on sexuality is a fairly new topic of discussion because it is quite difficult and confusing. This article will explore different gender, sex and sexual orientation identities and the main biological reasons behind these. There is also some validity of different sexual orientations and identities through evidence of sexual disorders such as Klinefelter and Turner syndrome and gender dysphoria. To discuss the biology of gender identity and sexual orientation, it is necessary to first examine the differences between multiple definitions that are often mistakenly interchanged: sex, gender, sexual orientation, and gender identity. Sexual orientation is defined by LeVay (2011) as “the trait that predisposes us to experience sexual attraction towards people of the same sex, towards people of the opposite sex or towards both sexes” (p. 1). The typical categories of sexual orientation are homosexual, heterosexual and bisexual. Vrangalova and Savin-Williams (2012) found that most people identify as heterosexual, but there are also groups of people who identify as predominantly heterosexual and predominantly gay within the three traditional categories (p. 89). This is to say that there are not three concrete groups, but sexual orientation is a continuum and can even fluctuate over time. LeVay (2011) also defines gender as “the… focus of the article……viewed October 9, 2015 from http://www.mayoclinic.com/health/klinefelter-syndrome/DS01057.Swaab, DF & Garcia-Falgueras, A. (2009). Sexual differentiation of the human brain in relation to gender identity and sexual orientation. Functional Neurology, 24(1), 17-28. Turner Syndrome Society. (2011). Further information on TS: fact sheet. Turner Syndrome Society of the United States. Retrieved October 9, 2015, from http://turnersyndrome.org/learn-about-ts/fact-sheetVrangalova, Z., & Savin-Williams, R. C. (2012). Mostly heterosexual and mostly gay/lesbian: Evidence for new sexual orientation identities. Archives of Sexual Behavior, 41, 85-101. doi: 10.1007/s10505-012-9921-y. Zhang, S., Liu, Y., & Rao, Y. (2013). Serotonin signaling in the brains of adult female mice is necessary for sexual preference. Proceedings of the National Academy of Sciences, 110(24), 9968-9973.