Without question, the greatest challenge to modern cosmology is understanding the nature and origin of both the cold dark matter responsible for most of the gravitational mass of galaxies and clusters and the dark energy responsible for the present apparent cosmic acceleration. This talk will review the current observational evidence and for these components and review various views on their origin. The simplest particle physics explanations for the cold dark matter is, perhaps, that of the lightest supersymmetric particle, an axion, or a heavy (e.g. "sterile") neutrino. The dark energy, on the other hand is generally attributed to a cosmological constant, a vacuum energy in the form of a "quintessence" scalar field possibly very slowly evolving along an effective potential. However, the simple coincidence that both of these unknown entities currently contribute comparable mass energy toward the closure of the universe begs the question as to whether they could be different manifestations of the same physical phenomenon. In this talk we review a variety of proposals whereby the dark matter and dark energy may be related. Examples include bulk viscosity, relativistic corrections due to an inhomogeneous distribution of dark matter, or the flow of dark matter from higher dimensions in in brane- world cosmology.