Curl and cockle are discussed, having reference respectively to (a) deviation from the flat form resulting from inequities in structure in the thickness dimension from one face of the paper to the other, and (b) localized distortions out of the flat form resulting from inequalities from area to area of the paper. The fundamental mechanism of curl is discussed. The most important factor in curl is the differential in fiber orientation through the thickness of the sheet. The net difference between the two faces in this respect is shown to be li? early related to the degree of curl encountered. The effect of other inequalities in structure and composition through the thickness of the sheet is discussed. The effects of other factors, especially type of pulp and extent of beating are outlined. Curl is favored toward the side of the paper dried last. The basic reason for this is discussed in some detail. Compensation for the curl caused by differential fiber orientation through the thickness may be made by suitable differences in dry? temperatures, affecting the two sides of the sheet. In nearly all cases, the form of curl id either (a) toward the wire side with axis in the machine direction, or less commonly (b) the complementary form of curl toward the top side with axis in the cross direction. The two remaining possible forms of rectangular curl are rarely encountered. The cause of angular or oblique curl is discussed. Cockle is brought about by localized inequalities in moisture content prior to drying. The earlier drying of less moist areas sets up dried-in strains which restrict further shrinkage, and these areas are then warped out of the plane of the paper on further drying of the more moist areas. The effect of beating on cockle is discussed.