HERMES is an experiment investigating the quark-gluon structure of matter: we study the spin structure of the nucleon. By nucleon, we refer to the protons and neutrons which are the fundamental components of all atomic nuclei. However, if you look a little closer, you find that these nucleons are not as fundamental as they might appear... in fact, they are bound states, composite systems composed of even more fundamental constituents, namely quarks and gluons, held together by the strong force. And all of these particles have an important property called spin. The quarks have spin 1/2, the gluons have spin 1, and the nucleon itself has spin 1/2. The question we seek to answer at HERMES goes roughly like this: "How do the constituents of the nucleon conspire to produce its overall spin of 1/2?" We certainly know some things, but we haven't yet found the answer to that question. HERMES data-taking started in 1995 and ended in summer 2007.