In steady gas-liquid flow in homogeneous porous media with surfactant present, there is often observed a critical injection velocity or pressure gradient delp(min) at which "weak" or "coarse" foam is abruptly converted into "strong foam", with a reduction of one to two orders of magnitude in total mobility: i.e., "foam generation". Earlier research on foam generation is extended here with extensive data for a variety of porous media, permeabilities, gases (N-2 and CO2), and surfactants. For bead and sandpacks, delp(min) scales like (1/k), where k is permeability, over 2 1/2 orders of magnitude in k; for consolidated media, the relation is more complex. For dense-CO2 foam, delp(min) exists but can be less than 23 KPa/m (1 psi/ft). If pressure drop, rather than flow rates, is fixed, one observes an unstable regime between stable "strong" and "coarse" foam regimes; in the unstable regime delp is nonuniform in space or variable in time. Results are interpreted in terms of the theory of foam mobilization at a critical pressure gradient.