Some implications of μSR experiments on high temperature superconductors are discussed. The possible relationship between the observation of spin freezing and the tendency of mobile holes to phase separate is considered. Measurements of the magnetic penetration depth as a function of doping are shown to imply that a non-retarded, or high energy scale pairing interaction is responsible for high temperature superconductivity. The depression of the transition temperature at high doping is associated with a crossover away from the regime in which a charge-transfer gap is relevant.