The relation of fiber ultrastructure to the mechanical properties of wood demonstrates the constraints that structure and properties place upon each other. Fiber and other plant cell walls are composite materials in that they consist partly of a matrix of various amorphous, branched, or short-chain polymers, and partly of a well-oriented microfibrillar framework. Models for the mechanical behavior of fibers and wood must be revised to conform with this composite structure; similarly, certain structural proposals must be eliminated because they conflict with our increased knowledge of the mechanics of such materials. The application of crystal physics and solid mechanics to obtain more precise estimates of the mechanical properties of the matrix and framework components of fiber walls is essential. Examples are given for the determination of matrix and framework elastic constants.