  In these lectures, superconductivity in impure thin films close to the
absolute zero of temperature is discussed. The behavior as function of the
applied magnetic field and the amount of impurities suggests the presence of a
superconductor-insulator transition at zero temperature. The theory of
superconductivity in the limit where all the electrons become tightly bound in
pairs is used to explain the main characteristics of the transition. In that
limit, where the theory becomes equivalent to a phase-only theory, electron
pairs exist on either side of the transition. The presentation is pedagogical
in nature and includes exercises as a learning aid for those new to the field.
