Ordered and disordered states in magnetically diluted insulating systems

The different kinds of magnetic phases arising in insulating systems are described and discussed. Attention is focused on semi-disordered phases which retain partially long range order as reentrant properties and local canted states, and disordered phases without long range order as spin glasses and spin cluster states. The properties of some systems are described, choosen among those where Moessbauer spectroscopy has been performed, mainly spinels, glasses, fluorides, Eu-based compounds. Some unsolved problems are shortly pointed out. (orig.)