Cataracts can be defined as any opacity of the crystalline lens, often associated with breakdown of the lens microarchitecture, possibly including vacuole formation and disarray of lens cells, which can cause large fluctuations in density resulting in light scattering. In addition, light scattering and opacity will occur if there is a significant amount of high molecular weight protein aggregates. Cataracts can be classified by the age at onset: a congenital or infantile cataract presents within the first year of life; a juvenile cataract presents within the first decade of life; a presenile cataract presents before the age of about 45 years, and senile or age-related cataract after that. Congenital cataracts are a major cause of induced blindness in children, and inherited cataracts are the major cause of congenital cataracts. Inherited congenital cataracts have been associated with mutations in specific genes, including those of crystallins, gap junction proteins, membrane transport and channel proteins, the cytoskeleton, and growth and transcription factors. Inherited congenital cataracts may be inherited as autosomal dominant (most frequent), autosomal recessive, or X-linked traits.