Sex chromosomes, particularly the X chromosome, play a unique role in evolution due to several distinct features. In Drosophila, the X chromosome has been proposed to constitute an undesirable environment for genes expressed at higher levels in males than in females (ie male-biased genes) and, as a result, is partially demasculinized. However, male-biased genes remaining on the X chromosome do not seem to be at a disadvantage relative to their autosomal counterparts. Population genetic models predict that under certain conditions X-linked genes will experience more bouts of positive selection than autosomal genes, leading to faster-X evolution, particularly for male-biased genes. As theory posits, Drosophila X-linked male-biased genes show evidence of adaptive evolution at both protein and expression levels. This faster-X evolution has broad implications. In speciation, it may contribute to explain why the X chromosome is a hotspot for the genetic factors underlying hybrid male sterility (ie the large X-effect).