Interfacial Instabilities must abound in the ISM.They arise naturally in colliding gas flows,as well as in the proximity of velocity and density discontinuities.In general,such instabilities will be of hybrid variety, including Rayleigh Taylor,Kelvin-Helmholtz and (Jeans) self-gravitational terms.In addition,a new self-gravity term arises if a density discontinuity exists.The linear development of such instabilities will be discussed,and the nonlinear growth of particular cases illustrated.Although our understanding their nonlinear development is limited,it will be argued that these instabilities can initiate gravitational collapse on roughly a free fall timescale.