In this book some 50 papers published by K A Müller as author or co-author over several decades, amplified by more recent work mainly by T W Kool with collaborators, are reproduced. The main subject is Electron Paramagnetic Resonance (EPR) applied to the study of perovskites and other oxides with related subjects. This wealth of papers is organized into eleven chapters, each with an introductory text written in the light of current understanding. The contributions of the first author on structural phase transitions have been immense, and because K A Müller and J C Fayet have published a review paper on the subject, the latter is reproduced in chapter VII. Not related to EPR is a part of chapter VIII on the dipolar and quantum paraelectric behavior with dielectric studies. In chapter X two papers proving the existence of Fermi glasses are reproduced.The year 1986 bears some significance: early in this year the paper by Bednorz and Müller on the possible observation of superconductivity was published. This resulted in a substantial shift in the paradigm of condensed matter physics, to which the present first author has contributed scientifically with others and which will be reviewed in a second volume.