Low Mach number equation sets approximate the equations of motion of a compressible fluid by filtering out the sound waves, which allows the system to evolve on the advective rather than the acoustic time scale. Depending on the degree of approximation, low Mach number models retain some subset of possible compressible effects. In this talk I will discuss low Mach number models for reacting and stratified flows arising in combustion, astrophysics and atmospheric modeling.