An electron-paramagnetic-resonance (EPR) study dealing with the means for introducing substitutional N into silicon and the structure of N centers is presented in this paper. Nitrogen can be introduced into crystalline silicon by N+ implantation and subsequent pulsed-ruby-laser annealing. The primary criteria for introducing substitutional N in silicon are that enough energy be supplied during pulsed-ruby-laser annealing to melt the implanted region and that crystalline silicon regrowth occurs. The fraction of the implanted N observed by EPR to be incorporated into substitutional sites (labeled SL5) was measured as a function of N+ implant energy, N+ fluence, and laser annealing energy, and was found to be <~ 10%. Residual implantation lattice damage is tentatively believed to enhance the formation of other N-defect centers (SL6, SL7) upon thermal annealing between 400 and 450°C. Unlike the other group-V impurities (P, As, Sb, Bi), substitutional N is observed to be a deep-level impurity with axial symmetry about a <111> (C3v symmetry) direction. The characteristic reorientation time of the SL5 center is reasonably well described by the Arrhenius expression τ=τ0exp(EkBT) over the measured temperature region 35<~T<~200 K with E=0.107+/-0.02 eV and τ0~3×10-12 sec. The reorientation of the SL5 center is enhanced under illumination at low temperatures by (near-) band-gap incandescent light. Contrary to our prior assertion, the position of the N-donor level in the band gap remains unknown. The axial symmetry of the substitutional N center is attributed to a distortion in the positions of the substitutional N and neighboring silicon atoms. Possible explanations for this distortion are reviewed.