The advantages of using the multifrequency (3-cm, 8-mm, and 2-mm) EPR method in combination with the modified method of spin label and spin probes, the methods of steady-state microwave saturation and microwave saturation transfer of spin packets in the studies of the spin and electrodynamic properties of polyaniline are outlined. The methods of determining the composition of localized and delocalized paramagnetic centers and calculating their main parameters from total ESR spectra are described. Original results of investigations of polyaniline modified with sulfuric, hydrochloric, camphorsulfonic, and 2-acrylamido-2-methyl-1-propanesulfonic acids are reported. The dependences of the nature, electronic relaxation, dynamics of paramagnetic centers, and the charge-transfer mechanism on the method of synthesis, the structure of the acid molecule, and the polyaniline oxidation level are shown