Modelling dark energy - from a cosmological constant to quintessence to modified theories of gravity

I briefly review models of dark energy. These include models of a cosmological constant including those arising in string theory, and dynamical models where a scalar field may be responsible for the observed late time acceleration, through to the possibility that we are not fully in control of the gravity sector and the acceleration may be some manifestation of modified gravity. All cases require some degree of fine tuning for the models to be compatible with observation.