The high electric resistivity of the normal phase of high Tc superconductors, together with non-conventional scaling laws, are some of the hallmarks of 'bad metals'. Bad metallic behaviour presents a longstanding conundrum to theory. That diffusive processes control transport provides an appealing proposal to explain such universality. In this talk, I will explain how black hole horizons know about diffusion and provide simple ways of computing diffusive conductivities as well as elaborate general scaling theories, which may be applicable to bad metals.
