  One of the most relevant manifestations of the
Beresinskii-Kosterlitz-Thouless transition occurs in quasi-two-dimensional
superconducting systems. The experimental advances made in the last decade in
the investigation of superconducting phenomena in low-dimensional correlated
electronic systems raised new questions on the nature of the BKT transitions in
real materials. A general issue concerns the possible limitations of
theoretical predictions based on the XY model, that was studied as a
paradigmatic example in the original formulation. Here we review the work we
have done in revisiting the nature of the BKT transition within the general
framework provided by the mapping into the sine-Gordon model. While this
mapping was already known since long, we recently emphasized the advantages on
such an approach to account for new variables in the BKT physics. One such
variable is the energy needed to create the core of the vortex, that is fixed
within the XY model, while it attains substantially different values in real
materials. This has interesting observable consequences, especially in the case
when additional relevant perturbations are present, as a coupling between
stacked two-dimensional superconducting layers or a finite magnetic field.
