The effect of parental age on the characteristics of the offspring is considered. In man the overall incidence of congenital abnormalities increases with maternal age, although the first born may be slightly more prone to certain abnormalities than would be expected. In the mouse and guinea pig the incidence of certain abnormalities decreases with maternal age. An explanation is offered for the decrease. In several organisms, it is likely that minimum developmental variability occurs for offspring from mothers of intermediate age, which is usually the time of greatest reproductive capacity. Recombination fractions vary with maternal age in Drosophila melanogaster, and with maternal and paternal ages in the mouse. In man, non-disjunction increases with maternal age. In man some dominant mutations increase with paternal age; furthermore, paternal age is an important factor controlling the secondary sex ratio. Non-genic (cytoplasmic) factors can accelerate or decelerate aging in some organisms, e.g. rotifers. It is considered that the development of this approach will be important in aging research. Some plant evidence is briefly mentioned.