The discovery of neutrino oscillations has demonstrated that neutrinos have small, but non-zero masses. These masses are at least 10^6 times smaller than those of the other fundamental particles, suggesting that neutrinos could be getting their masses through a different physical mechanism. Understanding the fundamental nature of neutrinos and the mechanism by which they acquire mass are key goals of nuclear and particle physics in the coming years.
In this talk I will describe current and future searches for neutrinoless double beta decay with the Enriched Xenon Observatory (EXO). This extremely rare nuclear decay can occur if neutrinos are Majorana particles, i.e. if neutrinos and anti-neutrinos are identical. Searches for this lepton number violating decay will probe extensions to the Standard Model that attempt to account for the mechanism by which neutrinos acquire mass and could constrain the absolute neutrino mass scale. I will discuss the current status of the EXO experimental program and plans for next-generation detectors, which have substantial possibility to observe this beyond the Standard Model process.