  The nonlinear sigma model (NLSM) epitomises a field-theoretical approach to
(interacting) electrons in disordered media. These lectures are aimed at the
audience who might have vaguely heard about its existence but know very little
of what is that, even less so of why it should be used and next to nothing of
how it can be applied. These what, why and mainly how are the subject of the
present lectures. In the first part, after a short description of why to be
bothered, the NLSM is derived from scratch in a relatively simple (but still
rather mathematical) way for non-interacting electrons in the presence of
disorder, and some illustration of its perturbative usage is given. In the
second part it is generalised, not without some leap of faith, to include the
Coulomb repulsion and superconducting pairing.
