The authors of this contribution to the literature of resonance spectroscopy in paramagnetic systems are primarily concerned with the properties of the rare earth ions and, as such, the formal derivation of crystal field theory is set out in a manner which reflects this dominant interest. The ions of the 3d transition group are perhaps given too cursory a treatment in Chapter Two for those students of RF spectroscopy who have a somewhat less rare-earth oriented interest in the subject. Since the exam- ples cited in the text do include some 3d transition ions, it is perhaps worthwhile in a...