Estimates of the abundances of volatile, alkali elements (K, Rb, and Cs) in the bulk Silicate Earth vary considerably. The K and Rb abundances are constrained by the K/U (~ 1.3 × 104), K/Rb (~380), Rb/Sr (~0.03), and Ba/Rb (~11) ratios of the bulk Earth and by Sr, Nd, and Hf isotope systematics. The Cs abundance of the Silicate Earth is constrained by estimates of the Rb Cs ratios of the continental crust and mantle. The continental crust has a Rb Cs ratio of about 25, whereas the depleted MORB source and OIB plume source regions have a Rb Cs ratio of about 80. There is evidence suggestive of a secular change in the Rb Cs ratios of the depleted mantle, which may have been caused by continental crust formation and crust-mantle recycling processes. The Rb Cs ratio in the Silicate Earth is estimated to be about 28, based upon studies of the continental crust, MORB source, and OIB (plume) source. The continental crust contains about 37% of the total K present in the Silicate Earth, 50% of its Rb, and 55% of its Cs, whereas the residual mantle (the MORB and OIB source) contains about 20% of the K, 10% of the Rb, and only 4% of the Cs. Together, these reservoirs account for only about 60% of the total inventory of K, Rb, and Cs in the Earth today, indicating the existence of a less depleted reservoir in the mantle that contains the remainder of these elements. The average Rb Cs ratio for all lunar samples is about 22 and this is believed to represent the bulk Rb Cs ratio of the Moon. Within the limits of uncertainty which apply to both estimates, this ratio is similar to that of the Silicate Earth. Hence, we conclude that the existence of significant difference in the Rb Cs ratios of the Earth and Moon cannot be inferred from the presently available data base. Thus, we disagree with the claim of Kreutzberger et al. (1986) that the Moon has a significantly lower Rb Cs ratio than the Earth.