The principal sources of errors in the determination of parameters of the spin hamiltonian by conventional electron paramagnetic resonance (E.P.R.) experiments are identified. It is possible to circumvent these errors by performing an E.P.R. experiment in zero magnetic field, scanning the frequency. The benefits of zero-field (electron paramagnetic) resonance (Z.F.R.) and of combining E.P.R. and Z.F.R. techniques in a variable-frequency, variable-field experiment are listed and discussed. Such an approach offers a challenge to theorists to calculate the higher-order parameters which can now be obtained with high accuracy. Experimental advantages are also discussed.