Аннотация:The results of EPR and ENDOR studies of Si nanocrystals in porous silicon layers are reviewed. The dominant type of paramagnetic centers in porous silicon, prepared in different conditions, is Si dangling bond at Si/SiO2 interface or Pb center. Pb center concentration is very sensitive to vacuum heating, oxidation and hydrogen treatment. The Pb-center − 1H, 19F atoms distances have been estimated in the dipole-dipole approximation. Silicon dangling bond, similar that in amorphous silicon, is observed in porous silicon layers during vacuum annealing at temperatures higher than 300-400 0C. Free electron paramagnetic states, EX and E/ center defects are also observed in this material. EPR spectroscopy can be successfully used for diagnostic of singlet oxygen generation in porous silicon layers.