RESUMEN Los sistemas de determinacion de sexo son extremadamente variados, razon por la cual, han constituido un campo de constante investigacion. Comprender como se determina el desarrollo gonadal y mas aun dilucidar los procesos alli implicados, ha requerido considerar y relacionar la informacion obtenida desde campos como la citologia, embriologia, morfologia, citogenetica, biologia molecular, evolucion y ecologia. Este manuscrito presenta informacion de los procesos de determinacion de sexo principalmente en mamiferos placentarios, desde diversos niveles de organizacion biologica, asi mismo discute la importancia de las herramientas moleculares a la luz de la conservacion genetica. ABSTRACT Systems of sex determination are extremely diverse, reason by which, it has became a constant research field. Understand “how is the decision made about the gonadal development and even more to elucidate actors involved there, has required to consider and relate the information obtained from areas such as cytology, embryology, morphology, molecular biology and even ecology and evolution. This review is related to the sex determination process mainly in placental mammals, from diverse levels of biological organizations.