Gravitational lensing is the effect in which light from distant sources is deflected by the intervening mass, resulting in distortions in observed images. Weak gravitational lensing, where this distortion is sufficiently small to be detectable only statistically, is a key cosmological observable, the measurement of which is a top priority of several current and next-generation cosmological galaxy surveys. With it, we can probe the evolution of dark energy, the nature of gravity, and other fundamental physical questions.