The dissipation of tidal friction in the oceans and atmosphere has been estimated by calculating the secular perturbations of the moon's orbit by the tides using the available tide models for the principal semidiurnal and diurnal tides. Only the second-degree wavelength components in the ocean tide cause significant secular changes in the lunar orbit. These components of the M2 tide agree within 15% among the different ocean tide models. Other frequencies (O1, N2, S2, P1, and K1) have effects on the lunar motion of about 25% of the M2 tide and on the earth's rotation of about 45% of M2. The computed lunar acceleration of -35 arc sec/(100 yr)2 agrees to better than 15% with nearly all recent astronomical determinations. There has been no significant change in the tidal effect during the last 2000 or 3000 years. The difference beween the two estimates of the lunar acceleration is of the same order as their uncertainties. Hence only upper limits can be set on the 1/Q of the solid earth and moon of about 1/50 and on dissipation in the core of about 3¿1018 erg s-1. The total tidal acceleration of the earth is estimated to result in a 3.7-ms/100 yr increase in the length of day. The most recent discussion of the ancient eclipse records indicates an observed increase in the length of day of only 2.5 ms/100 yr. If the nontidal acceleration is attributed to the postglacial response of the earth, the decay time is about 3000 years, which is equivalent to a mean viscosity for the mantle of about 1022 P. source.