Modern density-functional methods for the calculation of electronic g-tensors have been implemented within the framework of the deMon code. All relevant perturbation operators are included. Particular emphasis has been placed on accurate yet efficient treatment of the two-electron spin-orbit terms. At an all-electron level, the computationally inexpensive atomic mean-field approximation is shown to provide spin-orbit contributions in excellent agreement with the results obtained using explicit one- and two-electron spin-orbit integrals. Spin-other-orbit contributions account for up to 25-30% of the two-electron terms and may thus be non-negligible. For systems containing heavy atoms we use a pseudopotential treatment, where quasirelativistic pseudopotentials are included in the Kohn-Sham calculation whereas appropriate spin-orbit pseudopotentials are used in the perturbational treatment of the g-tensors. This approach is shown to provide results in good agreement with the all-electron treatment, at moderate computational cost. Due to the atomic nature of both mean-field all-electron and pseudopotential spin-orbit operators used, the two approaches may even be combined in one calculation. The atomic character of the spin-orbit operators may also be used to analyze the contributions of certain atoms to the paramagnetic terms of the g-tensors. The new methods have been applied to a wide variety of species, including small main group systems, aromatic radicals, as well as transition metal complexes.