An old-growth forest (also termed primary forest, virgin forest, primeval forest, late seral forest, or in Britain, ancient woodland) is a forest that has attained great age (and associated structural features), and thereby exhibits unique ecological features. Old-growth forests tend to have more large trees and standing dead trees, multi-layered canopies with gaps resulting from the deaths of individual trees, and coarse woody debris on the forest floor.

Old-growth forests are often biologically diverse, and home to rare species, threatened species, and endangered species of plants and animals, such as the northern spotted owl, marbled murrelet and fisher, making them ecologically significant. Levels of biodiversity may be higher or lower in old-growth forests compared to that in second-growth forests, depending on specific circumstances, environmental variables and geographic variables. Logging in old-growth forests is a contentious issue in many parts of the world. Excessive logging can reduce biodiversity, affecting not only the old-growth forest itself, but also indigenous species that rely upon old-growth forest habitat. Fallen timber, or coarse woody debris, contributes carbon-rich organic matter directly to the soil, thus providing a substrate for mosses, fungi and for seedlings, and in creating microhabitats by creating relief on the forest floor. In some ecosystems, such as the temperate rain forest of the North American Pacific coast, fallen timber may become nurse logs, providing a substrate for seedling trees.

Plant species that are native to old-growth forests may someday prove to be invaluable towards curing various human ailments, as has been realized in numerous plants in tropical rainforests.

Old-growth forests were often given harvesting priority because they have the most commercially valuable timber, they are considered to be at greater risk of deterioration through root rot or insect infestation, and they occupy land that could be used for more productive second-growth stands.

In 1950 Howard earned his Ph.D. in zoology at Yale University, under the guidance of G. Evelyn Hutchinson. His dissertation was titled The Biogeochemistry of Strontium: With Discussion on the Ecological Integration of Elements. This step took him from his early interest in ornithology and brought him into the emerging field of systems ecology. Through this a meteorologist "analysis of the global circulation of strontium, anticipated in the late 1940s the view of the earth as one great ecosystem."

While at Yale, Howard began his life-long collaborations with his brother Eugene Odum. In 1953, they published the first English-language textbook on systems ecology, Fundamentals of Ecology. Howard wrote the chapter on energetics which introduced his energy circuit language. They continued to collaborate, in research as well as writing, for the rest of their lives. For Howard, his energy systems language (which he called "energese") was itself a collaborative tool.

The Center for Wetlands at the University of Florida, founded in 1973 by Howard T. Odum, is the first of its kind in the world to especially study wetlands. From 1956 to 1963, Odum worked as Director of the Marine Institute of the University of Texas. During this time, he became aware of the interplay of ecological-energetic and economic forces. He then taught at the University of North Carolina at Chapel Hill, where he was in the Department of Zoology, and one of the professors in the new Curriculum of Marine Sciences until his move to the University of Florida in 1970. He then taught at the Environmental Engineering Sciences Department for 26 years until his retirement in 1996. He also started and directed the Center for Environmental Policy at the University of Florida and founded the University's Center for Wetlands in 1973, the first of its kind in the world that is still in operation today.