This dissertation covers several disparate subjects in string theory and quantum gravity. The first part focuses on the effective field theory of D-branes in string theory. The low energy field theory of a particular configuration of intersecting D3-branes is worked out. Toy models derived from this field theory can then be used to explicitly exhibit S-duality. Finally, effective brane calculations have consequences in cosmological string theory: they suggest the perturbative stability of certain well-known constructions of de Sitter string vacua by Kachru, Kallosh, Linde, and Trivedi. The second part focuses on better understanding quantum gravity. A bulk higher spin gravity theory is constructed from the renormalization group flow of a free vector model. A proposal is given to express bulk locality of a gravity theory in terms of the natural gauge structure of the boundary dual theory. Finally, some restrictions are developed for a class of gravitational states in 2+1 AdS that are of interest to the construction of natural quantum gravity operators.