  These notes contain a rapid overview of the methods and results obtained in
the field of propagation of waves in disordered media. The case of
Schr\"odinger and Helmholtz equations are considered that describe respectively
electrons in metals and scalar electromagnetic waves. The assumptions on the
nature of disorder are discussed and perturbation methods in the weak disorder
limit are presented. A central quantity, namely the probability of quantum
diffusion is defined and calculated in the same limit. It is then shown that
several relevant physical quantities are related to the return probability.
Examples are provided to substantiate this, which include the average
electrical conductivity, its fluctuations, the average albedo and spectral
correlations.
