The monograph presents the main ideas, computational methods and results of numerical studies of phase transitions and critical phenomena in pure and disordered spin systems. The monograph is based on a series of scientific works carried out by the authors in this direction over the past twenty years. The main attention is paid to the study of the effect of frozen disorder realized in the form of non-magnetic impurities on phase transitions and critical phenomena in spin systems described by Ising and Potts lattice models on various lattices using numerical Monte Carlo methods.