Recent observations of distant supernovae have provided evidence that the expansion of the universe is accelerating rather than slowing down. While the most straightforward explanation for these results is the existence of a nonzero cosmological constant, a plausible alternative is vacuum energy from the potential of a slowly rolling scalar field, a/k/a/ "quintessence". I will discuss what we can learn about the physics of such a field by considering experimental limits on its interactions with ordinary matter, and loopholes by which it can avoid detection.