This book describes our present knowledge of protons and neutrons. Experiments using high-energy electrons, muons, and neutrinos reveal that the proton is made up of point-like constituents, i.e. quarks. The strong forces that bind quarks together, gluons, are described in terms of the modern theory of quantum chromodynamics (QCD). Larger and newer particle accelerators provide an opportunity to see deeper into the proton and probe the interactions between quarks and gluons at shorter distances. An understanding of this detailed substructure and of the fundamental forces responsible is one of the keys to unraveling the physics of the structure of matter.