Electron spin resonance (ESR) spectra of impurity and intrinsic defects have been measured at 4.2–295 K in Nb-doped BaTiO3 single crystals and ceramics to clarify their role in the conductivity and positive temperature coefficient of resistivity (PTCR effect). The measurements revealed a small amount of Fe3+, Cr3+, and Mn2+ impurities, which change their valence state (for example, Cr5+→Cr3+, Mn4+→Mn2+) with increased Nb concentration due to the compensation of the excess charge of Nb5+ ions. Besides the 3d-metal impurities, several types of Ti3+ polaronic and possible fluctuon states, where electrons can be localized near ferroelectric domain boundaries, have been revealed as well. All of them are associated with Ti3+ lattice ions and not with Nb5+/4+ impurity, which apparently represents rather a very shallow donor level. The data obtained strongly support the polaronic origin of Nb-doped BaTiO3 conductivity at T≪300 K. Comparative investigations of ESR spectra in single crystals and ceramics of the same kind of BaTiO3 together with computer simulation allowed us unambiguously to ascribe complex ESR signals observed in ceramic samples to Cr3+, Mn2+, and Fe3+ i- ons and Ti3+ polarons and/or fluctuons. The role of manganese ions at grain boundaries in the PTCR effect is discussed as well.