The article describes the microphysical aspects of surface moisture, a problem known to all conservators. The relationship between relative humidity, atmospheric temperature, moisture mixing ratio, and dew point is first explicated briefly. The cases of unreactive, non porous surfaces and objects with open and internal pores are considered in turn. Condensation occurs on unreactive, non porous surfaces when the surface temperature reaches the dew point, so heating surrounding air will not prevent condensation until the object itself heats up. Condensation on porous surfaces takes place at relative humidities less than 100 percent and at temperatures greater than the dew point. Open and internal pores with small outlets behave differently. Internal pores favor condensation-evaporation cycles, the exact nature of which depend on the distribution of radii of curvature of the pores and the distribution of radii of curvature of the pore outlets. Intermediate relative humidities are most conducive to condensation-evaporation cycles. Internal condensation occurs first in cavities with the smallest radii of curvature, but is also affected by interaction with the external environment when fluctuations in temperature or atmospheric pressure cause displacement of water inside the pore necks.