Wetting and adhesion should be treated as inherently irreversible processes, because, in the process of joining and separating two surfaces, available energy is dissipated. In a system that undergoes inherently irreversible processes, the second law of thermodynamics is always satisfied and can offer no information regarding the system’s equilibrium state. Therefore, to determine the equilibrium wetting angle, the law of energy conservation should be used. As a result, the advancing contact angle is found to be intrinsically higher than the receding one, regardless of the surface heterogeneity, and the extent of hysteresis is dependent on the amount of energy dissipated in the process of wetting and dewetting. ©1995 American Institute of Physics.