The present conference, the fourth successive on this subject, was organized to commemorate the 75th birthday of Professor Wtodzimierz Trzebiatowski, one of the pioneers in the field of f-electron materials structure, particularly in the magnetism of actinides. This volume contains 64 papers presented at the conference held in Wroctaw, Poland, September 22-25, 1981. Twenty-one were invited talks. About 100 participants from 13 countries attended the meeting during four days of lecture presentation (te these two numbers have been constant for the last two conferences). The conference consisted of sessions devoted to the investigation of crystalline electric fields and structural effects by spectroscopic techniques, neutron diffraction, magnetic, thermodynamic and electrical measurements all over broad temperature, magnetic field and pressure ranges. Materials investigated included rare earth intermetallics, hydrides, diluted systems and actinides, and among them some exhibited singlet ground state behavior. The experimental results were supplemented by theory. It is our pleasure to mention those persons who helped us make the conference successful. The International Advisory Com- mittee included W.J.L. Buyers, B.R. Cooper, J.E. Crow, P. Fulde, A. Furrer, T. Kasuya, L. Kowalewski, G.R. Lander, R. Lemaire and D. Wohlleben. We thank them for valuable suggestions concerning invited speakers. We also wish to thank the co-workers of the In- stitute for Low Temperature and Structure Research of the Polish Academy of Sciences in Wroclaw, especially A. Baran, M. Grzebyk, K.