de Sitter spacetime plays a central role in modern (quantum) cosmology. Yet, there are various aspects of quantum field theory in de Sitter that we do not understand at the same level as we do Minkowski spacetime. In particular, perturbative calculations in de Sitter give rise to a variety of contributions that grow with time (secular divergences). We study how such terms can be interpreted using a renormalization group approach, which eventually culminates in an effective theory description of the long wavelength physics. We also introduce techniques to compute loop integrals in de Sitter, a problem for which the standard flat space QFT methods are inadequate. These methods complement and validate the EFT description, and allow us to calculate higher order corrections to the Stochastic Inflation framework for massless scalars.