I will start by introducing the two fundamental pillars of Cosmology: General Relativity and the Cosmological Principle. General Relativity will be constructed both in the geometrical as well as in the particle physics perspective. After introducing some of the prominent effective field theories of gravity I will show how the Cosmological Principle can be realized in the different theories. I will then discuss how different theoretical consistency checks can be applied on them for their scrutiny, like conditions coming from a UV completion and quantum corrections. I will argue that we will need to combine this theoretical scrutiny program with different cosmological observations in order to disentangle between different dark energy models and test fundamental properties of gravity. [please email a.held@imperial.ac.uk for zoom link or password]