TheHubble constant H0 is a particularly important parameter in FriedmannRobertson-Walker (FRW) cosmologies (e.g. the concordance model), because it sets the length and timescale of the universe. Cosmological distances are directly proportional to H−10 , and a large uncertainty in H0 implies that we are unable to precisely determine the distances to any distant extragalactic object. This has dramatic consequences on the estimation of other cosmological parameters such as the energy densities Ωm0 and ΩΛ0. Although the analysis of the WMAP data provides a well defined relation between Ωm0 and ΩΛ0 (Spergel et al., 2007, see Fig 2.4), the actual determination of these values depends on the value of H0.