water relationships, the subject of dimensional stability is discussed. This refers to lineal and areal changes in dimensions with little reference to thickness. The primary factor involved is the highly anisotropic intra-fiber shrinkage. This is communicated to the sheet through the inter-fiber bonding. The intra-fiber shrinkage involves a corresponding axial compression of crossing fibers. The onset of sufficient adhesion at crossing points required for the transmission of these forces to the sheet must precede full shrinkage, and this matter is discussed in some detail. Expansion of the sheet at higher RH is linearly related to shrink-age on drying. Shrinkage has reversible and irreversible components and the nature of these is discussed. Hygrostability and hydrostability differ in degree only. Drying under tension, as in machine-made papers, has important effects not only in restriction of shrinkage and subsequent expansion but also in the introduction of dried-in stains in the paper. The effect of various factors on dimensional stability, including particularly fiber orientation and beating are discussed. The main effect of dimensional instability is in the cross direction, and various proposed methods of improving dimensional stability are outlined. -- AATA