Reproductive barriers are often thought of as fixed between species, yet recently diverged species often have polymorphic reproductive incompatibilities. This lecture will focus on the genetic basis of variable hybrid male sterility in house mice. I will focus on two mechanisms— transmission ratio distortion of hybrid incompatibilities and the regulatory disruption of the X chromosome during spermatogenesis. I will also discuss the implications of this variation on gene flow in the house mouse hybrid zone, and the maintenance of species boundaries.