  Signed graphs are graphs whose edges get a sign $+1$ or $-1$ (the signature).
Signed graphs can be studied by means of graph matrices extended to signed
graphs in a natural way. Recently, the spectra of signed graphs have attracted
much attention from graph spectra specialists. One motivation is that the
spectral theory of signed graphs elegantly generalizes the spectral theories of
unsigned graphs. On the other hand, unsigned graphs do not disappear
completely, since their role can be taken by the special case of balanced
signed graphs.
  Therefore, spectral problems defined and studied for unsigned graphs can be
considered in terms of signed graphs, and sometimes such generalization shows
nice properties which cannot be appreciated in terms of (unsigned) graphs.
Here, we survey some general results on the adjacency spectra of signed graphs,
and we consider some spectral problems which are inspired from the spectral
theory of (unsigned) graphs.
