There is a missing energy problem in cosmology: the total energy density of the Universe, based on a wide range of observations, is much greater than the energy density contributed by all baryons, neutrinos, photons, and dark matter. Deepening this mystery are the recent observations of type 1a supernovae which suggest that the expansion rate of the Universe is accelerating. One possible resolution is the existence of a cosmological constant which fills this energy gap. However, a logical alternative is "quintessence," a time-dependent, spatially inhomogeneous, negative pressure energy component which drives the cosmic expansion. This lecture will serve as an introduction to the quintessence cosmological scenario.