We describe some recent developments in the efficient modelling of light-matter interactions in plasmonics and nanophotonics. In particular, we describe an efficient finite-difference time-domain (FDTD) method to compute regularized quasinormal modes in resonant photonic structures, which have a wide range of applications in classical and quantum nanophotonics, and present methods to compute disorder-induced localization modes (Anderson localization) in slow light photonic crystal waveguides.