  Sudden changes in the internal structure of stars, placed at the interface
between convective and radiative regions, regions of partial ionisation, or
between layers that have acquired different chemical composition as a result of
nuclear burning, often produce specific signatures in the stars oscillation
spectra. Through the study of these signatures one may gain information on the
physical processes that shape the regions that produce them, including
diffusion and chemical mixing beyond the convectively unstable regions, as well
as information about the helium content of stars. In this talk, I will review
important theoretical and observational efforts conducted over the years
towards this goal. I will emphasise the potential offered by the study of
acoustic, gravity, and mixed modes observed in stars of different mass and
evolutionary stages, at a time when space-based data is allowing us to build on
the knowledge gained from the study of the sun and white dwarfs, where these
efforts have long been undertaken, extending the methods developed to stars
across the HR diagramme.
