History of Conventional Spin Wave Theory.- Basic Issues of Renormalization Group (RG) Theory.- Universality.- Microscopic Processes.- Non-Relevant Magnons.- Crossover Phenomena.- Metastability of Universality Classes.- Relevant and Non-Relevant Interactions.- Temperature Dependence of the Magnon Excitation Spectra.- Magnetic Heat Capacity.- Experimental Verification of GSW Bosons.- Magnets With and Without Magnon Gap (Goldstone Mode).- Microscopic Details: Spin Structure, Site Disorder, Two Order Parameters.- The Critical Magnetic Behaviour.- Thermal Lattice Expansion and Magnetostriction.- The Total Energy Content.- Superconductivity.- Conclusions.