We summarize here, and detail with numerical examples, the Quasi-Normal Mode theory which has been developed in a recent series of papers dealing with classical and quantum plasmonics. We present the semi-analytical formalism capable of handling the coupling of electromagnetic sources, such as point dipoles or free-propagating fields, with various kinds of dissipative and dispersive resonators. Due to its analyticity, the approach is very intuitive, and very versatile and can be applied to canonical problems of quantum optics and sensing with nanoresonators.