The time dependence of the self-diffusion coefficient D(t) in porous media is investigated by Monte Carlo simulation of autocorrelation functions [f(t)f(0)], where f(t) is the force of interaction between a molecule and the surface at time t. At short times, D(t) is governed by the surface population of the molecules and the probability of their return to the surface. At times t>t*, where t* is the characteristic time for the autocorrelation function to converge to zero, the apparent dynamics of the molecules is completely determined by the geometry of the surface on the length scale of rootD(0)t*, where D-0 is the bulk self-diffusion coefficient. D(t) in this limit is the sum of a constant D-infinity=lim(t-->infinity)(t) and a time-dependent term (R) over bar (2)(p)/2d.t, where (R) over bar (2)(p) is the mean-squared size of an effective unit cell of the porous space and d is the dimensionality of the space. The meaning of tortuosity for self-diffusion is discussed.