  In these lecture notes prepared for the 11th Taiwan Spring School, Taipei
1997}, and updated for the Saalburg summer school 1998, we review the solutions
of O(N) or U(N) models in the large N limit and as 1/N expansions, in the case
of vector representations. The general idea is that invariant composite fields
have small fluctuations for N large. Therefore the method relies on
constructing effective field theories for these composite fields after
integration over the initial degrees of freedom. We illustrate these ideas by
showing that the large N expansion allows to relate the phib^2^2 theory and the
non-linear sigma-model, models which are renormalizable in different
dimensions. In the same way large N techniques allow to relate the
Gross--Neveu, an example of a theory with four-fermi self-interaction, with a
Yukawa-type theory renormalizable in four dimensions, a topic relevant for four
dimensional field theory. Among other issues for which large N methods are also
useful we will briefly discuss finite size effects and finite temperature field
theory, because they involve a crossover between different dimensions.\par
Finally we consider the case of a general scalar V(phib^2) field theory,
explain how the large N techniques can be generalized, and discuss some
connected issues like tricritical behaviour and double scaling limit. Some
sections in these notes are directly adapted from the work Zinn-Justin J.,
1989, Quantum Field Theory and Critical Phenomena, Clarendon Press (Oxford
third ed. 1996).
