Plasma data from Helios A and B, IMP, ISEE 3, Pioneer 10 and 11, and Voyager 1 and 2, together with an MHD simulation model, have been used to study on a global scale the temporal and radial evolution of the solar wind's thermodynamic properties (temperature T, number density n, and entropy s). The study has given the following results: (1) Near and inside 1 AU, T and n are negatively correlated almost all the time. Outside ≈5 AU, the T,n relation evolves to become positively correlated. (2) At 1 AU, T, p, n, and &bgr;i are dependent on the sunspot number. During the minimum of the sunspot cycle, the negative correlations between T and n are very strong. (3) The radial evolution of temperature and entropy shows that a heating process takes place for the heliospheric plasma. Between 1 and 10 AU, the entropy increases by ≈4¿10-16 erg/K/proton. In addition, from the combined Voyager 1 and 2 observations between 1 AU and 10 AU, the solar rotation average proton temperature was found to decrease as r-0.46¿0.09, a rate much slower than the r-4/3 rate predicted by the adiabatic expansion of the solar wind. We interpret this heating to be largely due to the cumulative effect of the shock process with increasing distance from the Sun. (4) The simulation model is also used to extrapolate the thermodynamic properties of the solar wind from Voyager 2 to the region of the outer heliosphere bounded by the termination shock. As the heliocentric distance increases by a factor of 10, the temperature decreases by a factor of 3, and the shock process raises the entropy by 4¿10-16 erg/K/proton. ¿ American Geophysical Union 1989