The interaction of bodies blurs the concept of independent particles. This book presents a way of accommodating the interaction in ensembles of many interacting fermions, like electrons in solids, or H e 3 at low temperatures. The theory of interacting fermions at zero temperatures is described, and its application to the quasiparticle picture is thoroughly investigated, with the aim of relating Landau's theory of the normal Fermi liquid to the quantum-mechanical interaction effects. The reader should have a background knowledge of quantum mechanics, statistical physics and quantum-field theory. The book derives the phenomenological interaction function of the normal Fermi liquid from the underlying fermion interaction, and presents specific calculations of the relevant quantities. In particular, the validity of the quasiparticle concept is investigated, and quantitative limits are given. An estimation of the ground-state energy and the chemical potential is presented, which is a long-standing problem in this phenomenological theory.