Quintessence is the scalar field introduced by cosmologists in an attempt to solve the biggest problem in cosmology today: Chemical elements and dark matter do not account for even half of the content of the observed universe. Different types of energy have been dominant during different epochs in the history of the universe. Matter causes the growth of the universe to decrease. The cosmological constant causes it to accelerate, while Quintessence is in between: it causes the expansion to accelerate but less rapidly than that of the cosmological constant. Currently the bulk of the universe is thought to be "dark energy" with the strange property of negative gravity; it repels rather than attracting bodies together. Einstein's equations of General Relativity means that the strength of a gravitational force is not just dependant on mass, but also on other sources of energy and on pressure. So how does Quintessence fit into this? Quintessence is one way that physicists believe they can produce a negative pressure. The true origins of this are not fully understood, however, one of the simplest models proposes a quantum field whose energy is varying so slowly that it looks like a constant vacuum energy. The major difference between quintessence and the vacuum energy, which is also a possible solution to the problem, is that the pressure produced by quintessence is considerably less negative than that of the vacuum energy. This means it does not accelerate the universe as strongly as vacuum energy does. At present the field theories and predictions of quintessence fit more accurately with the observational data than those of vacuum energy. However, it will ultimately be observational data that decides between the two.