Passage of light is bent by gravity of astronomical object, producing gravitational lensing phenomena. Gravitational lensing can generate additional images, magnify and distort images, or even delete images. Lensing by the universe as a whole, by galaxy clusters, by galaxies, by stars, and by dark matter produces variety of astronomically observable consequences. Depending on the angular scales of the lensing effect, gravitational lensing can be classified as macro-lensing and micro-lensing. Important cases of each category are reviewed.