Despite the seemingly vast possibilities offered by the string landscape, it has been argued that not all consistent low energy quantum field theories can be ultraviolet completed into string theory. The space of consistent low-energy effective theories which cannot be completed to a full theory with gravity was dubbed the "swampland". In this talk, we revisit this "swampland" proposal in theories with axion-like fields. We describe how quantum gravity can limit the axion field range, which is otherwise unconstrained in the low energy effective theories. These constraints have interesting implications to axion inflation models, and the prospects of such models in generating detectable primordial gravitational waves.