Polar crude oil components can adsorb on mineral surfaces and alter their wetting properties. Extrapolating from laboratory observations of wetting alteration to reservoir conditions is difficult because of the chemical complexity of the crude oils, the variability of mineral surfaces, and the presence of brine. It is essential to recognize that the polar components of crude oils can adsorb by several distinct mechanisms, depending on factors that include brine composition and the solvent quality of the oil for its asphaltenes. In this article, several important mechanisms are identified. These have been demonstrated by experimental observations of contact angles between pure fluids on flat surfaces after exposure to crude oil. The consequences of these mechanisms for wetting alteration by different crude oils in porous media are examined.