In cosmology and physics, Quintessence is a hypothetical energy field that accounts for "Dark Energy" and is also formulated to provide an explanation for the observation that our universe is experiencing an accelerating rate of expansion. This 'dark energy field' functions as a form of 'anti-gravity' which is the source of the force that causes our universe to accelerate as it expands.Some physicists go so far as to label this new dynamic energy density field as a "fifth fundamental force." It is the latest attempt to deal with the 'cosmological constant' that was postulated by Einstein in 1917 to determine a static universe. Since our universe is expanding at an accelerating rate, as confirmed by numerous observations since 1997, the cosmological constant is therefore non-zero. The dynamic energy field termed quintessence is the current replacement for the zero cosmological constant factor, thus contributing a negative pressure term in Einstein''s Field Equation.Approximately 68% of the mass-energy density of our universe is attributed to "dark energy." The introduction of a dynamic energy field attributed to dark energy is the basis for the current cosmological model known as the Lambda-CDM model.This book is designed to be a general overview of the topic and provide you with the structured knowledge to familiarize yourself with the topic at the most affordable price possible. The level of discussion is that of Wikipedia. The accuracy and knowledge is of an international viewpoint as the edited articles represent the inputs of many knowledgeable individuals and some of the most currently available general knowledge on the topic, based on the date of publication.