This classic opening gambit at the stereotypical drinks party always throws me. I have been a professor at a university for most of my life, so the easiest answer is that I teach. This is true, but it disguises the reality that much of my waking time has been concerned with research. If I admit this, then it becomes necessary to explain what I actually research. One of my pet subjects is pyrite. But if I let on that I research pyrite, my interlocutors look at me as though I am one of those wonderful beings who haunt the bowels of natural history museums as world experts on a rare species of toad. As with toads, most people in the world have heard of pyrite. They know it is a mineral or stone, and most know that it is also called fool’s gold, a familiar theme of moral tales and nursery stories. So the idea of someone studying pyrite is not altogether the stuff of IgNobel prizes. Within the time limits imposed by decent conversation I cannot explain that pyrite is the mineral that made the modern world. I cannot refer them to a book about it since there has not been one published about pyrite since 1725. This book is an attempt to rectify the situation. In it I contend that pyrite has had a disproportionate and hitherto unrecognized influence on developing the world as we know it today. This influence extends from human evolution and culture, through science and industry, to ancient, modern, and future Earth environments and the origins and evolution of early life on the planet. The book is aimed at making the subject accessible to the general reader. It is not a scientific monograph, since these handle only the science and are really directed at the converted: the high priests of the cathedral of science and technology and their aspirant novices. It is also not aimed at being a textbook in the conventional sense: textbooks are generally aimed at specific academic courses and ultimately pave the way for the students to understand the monographs.