In the first MSA "Short Course Notes/Reviews in Mineralogy," Prewitt and Rajamani (1974, 1976) authored a chapter, "Electron Interactions and Chemical Bonding in Sulfides," in which we focused on bonding and crystal structures of transition-metal sulfides. Today's presentation compares what we now know about the characteristics of iron-nickel sulfides with the situation described in the above chapter. In 1974 most of the crystal structures of known iron-nickel sulfides had been published, but there were several unsolved problems regarding metal-sulfide and metal-metal bonding, we had only a vague idea about the nature of departures from stoichiometry in pyrrhotite, no significant high-pressure structure determinations had been performed, and our ability to investigate spin crossover in iron sulfides was limited. Today a few new crystal structures are known, we have a better understanding of bonding, investigations at high pressure have provided new insight, and new x-ray spectroscopic techniques provide a way to determine spin state in the iron sulfides. However, much remains to be done and this area of research still appears to be a promising objective for young investigators.