We discuss the formulation, validation, and parallel performance of a high-order accurate method for the time-domain solution of the three-dimensional Maxwell's equations on general unstructured grids. Attention is paid to the development of a general discontinuous element/penalty approximation to Maxwell's equations and a locally divergence free form of this. We further discuss the motivation for using a nodal Lagrangian basis for the accurate and efficient representation of solutions and operators. The performance of the scheme is illustrated by solving benchmark problems as well as large scale scattering applications.