Chapter 9 calculates the superfluid density as a function of temperature in d-wave superconductors. The in-plane superfluid density varies linearly at low temperatures. The c-axis superfluid density, on the other hand, is predicted to behave as T5 in copper oxides at low temperatures if the gap function possesses the dx2-y2-wave symmetry. This peculiar T5-dependence of the superfluid density was confirmed by experiments. Both the in-plane and c-axis superfluid densities, however, vary quadratically with temperature if the impurity scattering becomes important. The superfluid response in a two-gap system is discussed and applied to explain the anomalous temperature dependence of the superfluid density in electron-doped high-Tc materials. Both the nonlinear and nonlocal effects on the superfluid density of d-wave superconductors are also discussed.