Mildly delignified wood holocellulose fibers show well‐preserved cellulose nanofibril (CNF) structure in the fiber cell wall. Fibers, paper, biocomposites, and compression‐molded fiber materials demonstrate excellent mechanical properties. Here, wood holocellulose fibers and corresponding CNFs are discussed with respect to nanostructure, mechanical performance, and advanced materials potential. Functionalization routes are discussed, as well as materials selection, nanoscience of recycling, and the embodied energy in cellulosic candidates for multifunctional structural materials.