The human lens is a uniquely transparent, biconvex, avascular structure responsible for focussing the image onto the retina. It is enclosed by a thick capsular basement membrane. The accommodative process in the lens is dependent upon the inherent elasticity of the lens capsule and the contractile property of the lens cells or fibers. In contrast to the ocular dimensions, the lens shows marked changes in size and weight with aging. Impairment in lens transparency and significant morphologic changes in the nucleus, cortex, and capsule occur with aging process. The optical zones of discontinuities of the lens result from light scatter through the various parts of the lens and are of great clinical significance in the assessment of cataract. The lens preserves its entire population of cells from fetal life until death and provides an excellent model for defining aging parameters of other ectodermally derived structures.