In recent years, a search for a single, universal cause of aging has been replaced by the notion that the aging phenomenon is sufficiently complex to be governed by several complementary processes. This situation stems to a large extent from a fact that aging may be considered at various levels, starting from whole populations, through individual organisms, tissues and organs, ending on particular cell types. This complexity has determined currently functioning division of aging theories into evolutionary and mechanistic. First group aims at answering the question "why do we age?" and determining a biological purpose of this process. Mechanistic theories, in turn, try to answer the question "how do we age?" in terms of direct reasons of adverse changes that appear in organisms with age. The aim of this paper was to collect and present the most important theories of aging, pointing--if possible--on reciprocal relationships between them.