Testing General Relativity with Cosmology

With the successes of observational cosmology, a new window has opened up on to gravitational physics. By carefuly measuring the morphology and growth of structure in the Universe it may be possible to constrain general relativity on a completely new range of scales. It also allows us to explore a plethora of modified gravity theories that have emerged as an attempt to explain the observational evidence for dark energy. I will discuss the challenges and approaches which are being taken in this new field.