  Research on 2D materials has been one of the fastest-growing fields in
condensed matter physics and materials science in the past 10 years. The low
dimensionality and strong correlations of 2D systems give rise to electronic
and structural properties, in the form of collective excitations, that do not
have counterparts in ordinary 3D materials used in modern technology. These 2D
materials present extraordinary opportunities for new technologies, such as in
flexible electronics. In this Review, we focus on plasmons, excitons, phonons
and magnons in 2D materials. We discuss the theoretical formalism of these
collective excitations and elucidate how they differ from their 3D
counterparts.
