Axions arise ubiquitously in theories of particle physics and quantum gravity and may play important roles in cosmology, from inflation and dark matter to dark energy. This raises a basic question: does quantum gravity constrain the allowed parameter space of axions, and can such constraints have observable consequences? I will discuss how Euclidean wormholes provide a surprising connection between these questions and quantum gravity. I will first review the axionic Weak Gravity Conjecture (aWGC), including the importance of its order-one coefficient for axion phenomenology, and then describe how Euclidean wormholes may give rise to this bound. I will discuss the perturbative stability of axion wormholes and their explicit realization in string theory, both in flat space and in AdS. Finally, I will present recent work showing that the aWGC can arise directly from the nonperturbative fragmentation of Euclidean wormholes into charged defects. Requiring such fragmentation to lower the Euclidean action fixes the action-to-charge bound and its order-one coefficient, and reveals a direct connection to the Imaginary Distance Bound. This suggests that quantum-gravitational constraints on axions can be understood as conditions on the non-perturbative stability of gravitational saddles, with implications for axion cosmology and the consistency of effective field theories.