Landau's fermi liquid theory: With four parameters I can fit an elephant, and with five I can make him wiggle his trunk! I have no intention of mocking Landau's theory by the quote of John von Neumann (Attributed to von Neumann by Enrico Fermi). I want to understand why we are saying this theory of Landau is remarkably successful. There are many parameters in the theory and I don't see it's beauty. I'm sure one with deep insight into this theory can help us to see in what sense it is successful?
 A: The wonderful thing about Landau Fermi liquid theory is that it is a general theory for any system which can be described by a Fermi surface at low temperatures and has fermionic excitations. For example, while most metals have complicated interactions, by integrating out the interactions below the fermi surface we arrive at an effective theory of fermionic excitations with a renormalized mass. This explains why metals are so well described by ideal fermi gasses as the concept of fermionic excitations above the Fermi surface still holds although there are interactions. A condition for the applicability is that the interaction does not cause instabilities, which ensures the concept of fermi surface survives.  For attractive interactions the Fermi liquid theory starts to breakdown and can lead to Cooper pairs, ie superconductivity. The most wonderful thing is that the effective mass and effective interaction (and other relevant effective parameters) can be derived from the microscopic theory, but this microscopic theory is not necessary once the value of these effective parameters are known. 
