The development of silicon-based nanoscale technology for the realization of single electron, single spin quantum devices demands deep donor-based systems to achieve a major breakthrough in the field: high-temperature operation. Here, we suggest that, despite some preparation difficulties, substitutional nitrogen in silicon (N Si) represents an interesting candidate for this purpose, being observable by electron paramagnetic resonance (EPR) at room temperature. We report a study of the nature and dynamics of substitutional nitrogen in silicon, the so-called SL5 paramagnetic center, by X-band continuous-wave EPR, complemented by pulsed EPR. Both natural and Si 28 isotopically enriched nitrogen-doped silicon samples have been used, the latter providing an improvement in the accuracy of the spin Hamiltonian parameters.