Freemartinism is one of the most commonly found intersex conditions in cattle, although it may also occur in small ruminants. The freemartin phenotype appears in a dizygotic twin pregnancy where one twin is a male and the other is a female. Due to precocious anastomoses between the placental vascular systems of the two fetuses, masculinising molecules reach the female twin and disrupt the normal sexual differentiation, whilst in the male the effects of this association are usually minimal. In cattle, this condition is observed in 90 to 97% of twin pregnancies. A freemartin is, by definition, a genetically female fetus masculinised in the presence of a male co-twin, giving rise to a sterile heifer. Genital tract defects with varying severity can be observed in freemartin animals, which often present suppression and disorganization of the ovary, originating a rudimentary or a testis-like gonad depleted of germ cells. The uterine horns may be hypoplastic or instead may be reduced to a cord-like structure suspended in the broad ligament. Anatomic continuity between the uterus and the vagina is frequently absent, and the existence of rudimentary vesicular glands is typical. The external genitalia commonly presents enlarged clitoris, small vulva and a prominent, male-like tuft of hair. As a rule, heifers born twin to a bull have to be considered sterile and should be identified as early as possible to cull them from replacement stock. Despite its limitations, freemartinism is currently diagnosed by physical examination, as karyotyping or blood typing is often considered an unnecessary expense. In cattle, twinning trend has a genetic background that has been associated to hormonal regulation in favor of double ovulations. However, the genetic determinant on the basis of twinning seems to have small importance when compared to environmental or managementassociated factors, particularly in dairy cows. In fact, in dairy animals, in particular in high milk producing cows, it has long been proven the increase of twin calvings due to the hormonal and metabolic disturbances in the energy balance early in the post-partum period.With increased incidence of twin births in cow it is reasonably expectable a small increase in the occurrence of freemartins at the farm levels. In this paper it is the intent to describe the gross and histopathological findings of freemartinism in cattle, using data gathered from a study at an abattoir (17 cases) and from 3 cases diagnosed in living animals, supported by a review of the pathophysiology of the process, and to discuss the available methods for identification of freemartin animals at farm level.