The aim of this chapter is to present recent progress made by experimental EPR studies in the field of localized JT clusters. Several reviews and books treated this subject before. Avoiding to a maximum the duplication of matters already treated in detail there on the one hand and the task to make the chapter reasonably self-sufficient on the other hand are two aims which are not easily reconciled. The chosen compromise does not totally avoid some repetitions as regards the models of the JT effect. The conventional theory of EPR was totally left aside in this chapter because it would hardly be possible to give an understandable account of the EPR theory in this short chapter. There are several recent excellent books available concerning this theory.