ABSTRACT
For real fermion systems, the particle interaction and the exclusion principle act simultaneously; we are thus led to study degenerate Fermi liquids, in which both effects are important. This chapter explores Landau's semiphenomenological point of view and the applications of the theory to neutral systems such as degenerate liquid. It focuses on the Landau theory of Fermi liquids, which shows clearly all the important features brought about by the interaction. Landau's theory corresponds to an expansion in powers of the fraction of excited particles. Within the two restrictions, the theory is rigorous. It lends itself to a large number of problems: equilibrium properties, transport properties, response to long wavelength external perturbations. Landau's theory is not a "universal panacea": its well-defined limitations should always be kept in mind. Moreover, it uses (and provides) far more information than any experiment will ever sample. The chapter concludes with a brief summary of the range of validity and main achievements of Landau's theory.
