  The particle-hole excitation spectrum for doped graphene is calculated from
the dynamical polarizability. We study the zero and finite magnetic field cases
and compare them to the standard two-dimensional electron gas. The effects of
electron-electron interaction are included within the random phase
approximation. From the obtained polarizability, we study the screening effects
and the collective excitations (plasmon, magneto-excitons, upper-hybrid mode
and linear magneto-plasmons). We stress the differences with the usual 2DEG.
