The warming at the Paleocene-Eocene boundary about 55 million years ago is the subject of intense research, as it has the potential to inform us about the effects of warming on the global ecosystem. Despite many years of research, many questions remain regarding the specifics and dynamics of this transitionally warm period known as the Paleocene-Eocene Thermal Maximum (PETM). The proposed source of this warming is a large increase in methane, carbon dioxide, or both, possibly from volcanic activity, methane hydrates buried along the continental slopes, and methane emissions from wetlands. Global climate models adapted with Eocene geography and high greenhouse gas levels have so far been unable to reproduce the warm climate of the high latitudes depicted by proxy data from this time. The integration of proxy data derived from the geologic and fossil record with model output is also a challenge, and requires cooperation of scientists from a broad array of disciplines.