Mean field theory (MFT, also known as self-consistent field theory) is a method to analyse physical systems with multiple bodies. A many-body system with interactions is generally very difficult to solve exactly, except for extremely simple cases (random field theory, 1D Ising model). The n-body system is replaced by a 1-body problem with a chosen good external field. The external field replaces the interaction of all the other particles to an arbitrary particle. The great difficulty (e.g. when computing the partition function of the system) is the treatment of combinatorics generated by the interaction terms in the Hamiltonian when summing over all states. The goal of mean field theory is to resolve these combinatorial problems. MFT is known under a great many names and guises. Similar techniques include Bragg-Williams approximation, models on Bethe lattice, Landau theory, Pierre-Weiss approximation, Flory–Huggins solution theory, and Scheutjens–Fleer theory.

Die Molekularfeldtheorie (engl. mean field theory) ist eine Näherung, die Systeme von miteinander wechselwirkenden Teilchen als Systeme freier Teilchen in einem externen Feld betrachtet. Das externe Feld wird dabei als konstant angesehen und berücksichtigt somit nicht, dass jedes Teilchen durch sein Verhalten das Feld lokal verändert (d.h. Fluktuationen werden vernachlässigt).

Em mecânica estatística, teoria de campo médio é a teoria que estuda um problema de muitos corpos interagentes substituindo a interação por um campo externo apropriadamente escolhido. Desta maneira, toda a interação que um corpo sente devido aos outros é representada por um campo efetivo. Todas as flutuações são desprezadas, o que não permite que a teoria de campo médio seja mais do que uma aproximação de ordem zero.