Weak gravitational lensing by voids

We consider the prospects for detecting weak gravitational lensing by under densities (voids) in the large-scale matter distribution. We derive the bas ic expressions for magnification and distortion by spherical voids. Cluster ing of the background sources and cosmic variance are the main factors that limit in principle the detection of lensing by voids. We conclude that onl y voids with radii larger than similar to 100 h(-1) Mpc have lensing signal -to-noise ratio larger than unity.