An explanation for the late--time acceleration of the universe remains one of the greatest theoretical challenges in modem physics. In this Thesis we investigate a class of scalar field theories which have been proposed to provide an explanation to this problem. We shall consider the cosmological and observational constraints which can be placed on a wide class of scalar field theories. We investigate models of coupled neutrino quintessence (also known as mass-varying neutrino, or MaVaN, models), and provide cosmological constraints for this theory. We also consider the issue of stability in such models. The cosmological implications of theories of coupled quintessence are also considered, in which we allow for the existence of multiple scalar fields and many species of dark matter, and find that current observational bounds on such theories could be relaxed. We also show how such theories could in principle be differentiated from the standard single field case. In the final Chapter we will consider an alternative model for dark energy, namely modified gravity, and discuss the local and cosmological constraints which can be placed on the theory. In.particular, we will discuss the difficulty faced in trying to construct a stable, well--motivated alternative to General Relativity which satisfies local and cosmological observational constraints.