  A Green function analysis has been developed for quasiparticle spectrum and
localized states of a 2D graphene sheet in presence of different types of
substitutional disorder, including vacancies. The anomalous character of
impurity effects in this system is demonstrated, compared to those in well
known doped semiconductors, and explained in terms of conical singularities in
the band spectrum of pure graphene. The criteria for appearance of localized
states on clusters of impurity scatterers and for qualitative restructuring of
band spectrum are established and a phase diagram in variables ``disorder''
\emph{vs} ``electron density'' is proposed.
