Plasmons in metal surfaces and clusters have been extensively studied due to their potential applications in sensing, imaging, light harvesting and optical metamaterials. Graphene is a semimetal with tunable conductivity and hence can support plasmons as well. In addition to the tunability, graphene plasmons have relatively weak damping due to the high carrier mobility. In this talk, I’ll present our recent progress on the studies of plasmon excitations in graphene micro- and nano-structures and their behavior in an external magnetic field. The collective motion of Dirac fermions, which are relativistic with zero rest mass, shows peculiar properties with a tunable “plasmon mass”. We showed strong light-matter interaction in the terahertz frequency regime and demonstrated graphene plasmonic terahertz filters and polarizers with graphene/insulator stacks. Localized plasmons in graphene nano-structures go beyond terahertz frequencies. As an atomically thin plasmonic material which is sensitive to the surrounding environment, strong coupling between plasmons in graphene and substrate surface polar phonons has been unambiguously identified in the mid-infrared regime. A new plasmon damping channel through the emission of graphene optical phonons has been revealed for high frequency plasmons. Our study paves the way for graphene applications in photonics, optoelectronics and metamaterials, especially in the terahertz frequency regime.