The lack of sexual dimorphism in nestling, juvenile or adult birds of large number of avian species, makes it difficult or impossible sex determination based on phenotipic characteristics. To use molecular markers for bird sex determination is a rapid and safe procedure; moreover this methodology minimizes the stress during sampling, compared to other invasive techniques that could affect the health or biological stability of the animal. The early sex determination in birds is of particular importance when considering ex situ conservation programs, production, exploitation or population ecology studies. This review presents the methodologies used to sex determination, making emphasize on molecular tools, showing its advantages and limitations