  Understanding the reason for the observed accelerated expansion of the
Universe represents one of the fundamental open questions in physics. In
cosmology, a classification has emerged among physical models for the
acceleration, distinguishing between Dark Energy and Modified Gravity. In this
review, we give a brief overview of models in both categories as well as their
phenomenology and characteristic observable signatures in cosmology. We also
introduce a rigorous distinction between Dark Energy and Modified Gravity based
on the strong and weak equivalence principles.
