  We consider the prospects for detecting weak gravitational lensing by
underdensities (voids) in the large-scale matter distribution. We derive the
basic expressions for magnification and distortion by spherical voids.
Clustering of the background sources and cosmic variance are the main factors
which limit in principle the detection of lensing by voids. We conclude that
only voids with radii larger than $\sim 100$ \hm have lensing signal to noise
larger than unity.
