The ancient Greeks speculated that it might be air, fire or water. A century ago, physicists felt sure it was the atom. Today, we believe that the deepest layer of reality is populated by a diverse cast of elementary particles, all governed by quantum theory. From this invisible, infinitesimal realm, everything we see and experience emerges. It is a world of wonder, yet it can be mystifying in its weirdness. The author presents a guide to its inhabitants and their strange behaviours- as well as some of the hypothetical particles that physicists still hpe to discover,