We describe applications of (perturbed) conformal field theories to
two-dimensional disordered systems. We present various methods of study~: (i)
{\it A direct method} in which we compute the explicit disorder dependence of
the correlation functions for any sample of the disorder. This method seems to
be specific to two dimensions. The examples we use are disordered versions of
the Abelian and non-Abelian WZW models. We show that the disordered WZW model
over the Lie group \CG at level k is equivalent at large impurity density
to the product of the WZW model over the coset space \CG^C/\CG at level
(−2hv) times an arbitrary number of copies of the original WZW model. (ii)
{\it The supersymmetric method} is introduced using the random bond Ising model
and the random Dirac theory as examples. In particular, we show that the
relevent algebra is the affine OSp(2N∣2N) Lie superalgebra, an algebra with
zero superdimension. (iii) {\it The replica method} is introduced using the
random phase sine-Gordon model as example. We describe particularities of its
renormalization group flow. (iv) {\it A variationnal approach} is also
presented using the random phase sine-Gordon model as example. Lectures
presented at the '95 Cargese Summer School on "Low dimensional application of
quantum field theory".Comment: 41 pages, latex, uuencoded file with 2 figues include