Star formation proceeds inefficiently in galaxies for reasons that remain under debate. In the local ISM it is known that the cosmic rays (CRs) provide a significant fraction of total ISM pressure and therefore contribute to hydrostatic balance. We set out a model for the dynamical effect of CRs, directly accelerated as a result of star formation itself, on the ISM gas column. On the basis of this model, we will explain how CR feed-back sets an ultimate limit to the star formation efficiency of `ordinary' galaxies (normal spirals and dwarfs). Interestingly, most such galaxies -- including the Milky Way -- have star formation efficiencies approaching the maximum allowed by cosmic ray feed-back, suggesting they exist in a state of delicate, dynamically-determined equilibrium. However, at the higher surface densities pertinent to star burst systems pionic losses imply that CRs are dynamically unimportant on global scales while, at the same time, guaranteeing that such galaxies are luminous gamma-ray sources. In passing, we explain how our model leads to new insights about the observed GeV to TeV spectra of local starbursts.