  This paper surveys and gives a uniform exposition of results contained in
papers published by the team of authors.
  The subject is degenerations of surfaces, especially to unions of planes.
More specifically, we deduce some properties of the smooth surface which is the
general fibre of the degeneration from combinatorial features of the central
fibre. In particular we show that there are strong constraints on the
invariants of a smooth surface which degenerates to configurations of planes.
  Finally we consider several examples of embedded degenerations of smooth
surfaces to unions of planes.
  Our interest in these problems has been raised by a series of interesting
articles by Guido Zappa in 1950's.
