The paper addresses the defect structure of microcrystalline silicon, muc-Si:H. Electron spin resonance (ESR) techniques are employed to study the nature and energy distribution of paramagnetic states in a large variety of undoped and doped muc-Si:H samples prepared by various methods under different deposition conditions. A qualitative model for the density of states distribution in the energy gap is developed which is dominated by two kinds of dangling bonds and bandtail states at both band edges. In phosphorus-doped samples, ESR reveals a metal-insulator transition at a phosphorus concentration of 4 x 10(18) cm(-3). Light-induced ESR and electrically detected magnetic resonance show that at low temperatures recombination is dominated by tunneling transitions from the bandtail states into neutral dangling bonds. Apparently, at higher temperatures, direct capture from conducting states prevails. The results are discussed considering the heterogeneous structure of this material. (C) 2002 Elsevier Science B.V. All rights reserved.