Moisture in the atmosphere plays a substantial role in the deterioration of works of art and museum objects. In this bulletin, the relative humidity or Rh, a measure of the moisture content of air, is described and the part it plays, sometimes in conjunction with atmospheric pollutants, is discussed in terms of the cracking and flaking of paint layers, the shrinking of wood, the cockling of parchment and paper, the corrosion of metal, the growth of moulds and fungi, etc. A range of acceptable Rh values is suggested for museums. Some of the equipment available for monitoring relative humidity is described and the techniques available for controlling it are briefly discussed. -- ICCROM