Lake ontogeny refers to the developmental history of lake ecosystems. Two important processes associated with lake ontogeny include (1) infill of shallow water bodies with sediment and their transformation into bogs or terrestrial ecosystems, and (2) temporal changes in the trophic status (productivity) of lakes and associated shifts in the community (biological succession). Lake ontogeny is studied by comparison of contemporary environmental variables in lakes of different ages, or by paleolimnological (sediment history) techniques that permit reconstruction of past lacustrine conditions. A long-standing paradigm in the study of lake ontogeny is that lake ecosystems evolve from an early primitive state of oligotrophy to a later advanced state of eutrophy. This proposed progression has been attributed to the long-term accumulation of sediment, which ultimately reduces the hypolimnetic volume, thus promoting deep-water anoxia and enhanced internal nutrient cycling. Changes in the supply of nutrients from the watershed and accumulation of nutrients in the lake have also been implicated in altering trophic status. Evidence now shows, however, that lakes do not always follow a consistent path from oligotrophy to eutrophy. Rather, trophic state and associated environmental variables fluctuate through time in response to changes of numerous and sometimes interacting external variables such as climate, geology, soils, vegetation, and lake morphometry.