This overview is an attempt to throw a fresh look at the popular free radical theory of aging (referred to also as oxidative stress theory) which holds that the progressive decline in physiological functions is a result of accumulation of diverse deleterious changes caused by reactive oxygen species (ROS). We discuss the role of mitochondria as a major source of ROS in the cell and how these link accumulation of oxidative damage to the age-related changes in physiologic functions. The free radical theory of aging is analysed here from two different views of aging--one (the pessimistic view) that regards aging as the inevitable result of life activity the consequences of which are accumulation of errors in the genome and damage of the biomolecules, and the other (the optimistic view) which considers that it is the changes in mitochondrial pathways of apoptosis with age that cause the functional tissue changes and aging. We also discuss the possibility of delaying the aging process by appropriate diet or drug therapy, which includes also calorie restriction as a mechanism of modifying the generation of free radicals and body metabolism and thus extending lifespan as a result.