  This lecture is a tutorial introduction to coherent effects in disordered
electronic systems. Avoiding technicalities as most as possible, I present some
personal points of view to describe well-known signatures of phase coherence
like weak localization correction or universal conductance fluctuations. I show
how these physical properties of phase coherent conductors can be simply
related to the classical return probability for a diffusive particle. The
diffusion equation is then solved in various appropriate geometries and in the
presence of a magnetic field. The important notion of quantum crossing is
developed, which is at the origin of the quantum effects. The analogy with
optics is exploited and the relation between universal conductance fluctuations
and speckle fluctuations in optics is explained. The last part concerns the
effect of electron-electron interactions. Using the same simple description, I
derive qualitatively the expressions of the Altshuler-Aronov anomaly of the
density of states, and of the correction to the conductivity. The last part,
slightly more technical, adresses the question of the lifetime of a
quasiparticle in a disordered metal.
