Information concerning the mechanics of rock folding based on theoretical considerations, laboratory experiments, and field observations is presented in this paper. Theoretical considerations are based on the theory of elastic stability. Photoelastic model experiments that illustrate the theory and qualitatively describe complex cases are included. The wave lengths of folds are compared with the dimensional arrangement of sedimentary strata. A plot of this relationship shows the wave length to be approximately equal to 27 times the thickness of the dominant member. The importance of the stratification of sedimentary rocks in determining the geometry of folds is emphasized. The concept of describing a sedimentary section in terms of structural lithic units is discussed.