Old growth forest, also called primary forest, ancient forest, virgin forest, primeval forest, frontier forest or (in the UK) Ancient Woodland, is an area of forest that has attained great age and so exhibits unique biological features. Old growth forests typically contain large live trees, large dead trees (sometimes called "snags"), and large logs. Old growth forests usually have multiple vertical layers of vegetation representing a variety of tree species and age classes. Forest regenerated after severe disruptions, such as clear-cut or fire is often called second-growth or regeneration until a long enough time passes that the effects of the disturbance are no longer evident. Depending on the forest, this may take anywhere from a century to several millennia. Hardwood forests of the eastern United States can develop old-growth characteristics in one or two generations of trees, or 150-500 years.

Old growth forests may be home to rare species which depend on this now-rare habitat, making them ecologically significant. Biodiversity may be higher or lower in old growth forests than in second-growth forests depending on specific circumstances. Logging in old growth forests is a contentious issue in many parts of the world.

Defining “old growth” Concepts of “old growth” vary widely and are not always easily applied or reconcilable with one another.

“Ancient Woodland” is a term used in the United Kingdom to refer specifically to woodland dating back to 1600 or before (in England and Wales), or 1750 (in Scotland). Before this, planting of new woodland was uncommon, so a wood present at these dates was likely to have developed naturally. By this definition Ancient Woodland may have had considerable artificial interference, the important characteristic being continuity of woodland on the land.

By contrast, in the US, “old growth” is often used to imply a forest has experienced little direct disruption during contemporary historical epochs and looks about as it would had Europeans not come to America. This criterion is difficult to apply, since it is often impossible to determine the history of human management (Euro-American or Native American). And, since landscapes are naturally dynamic, there can be no certainty what forests would look like now had pre-Columbian regimes been uninterrupted. While it is generally agreed that old forests defined as “old growth” have not been subject to logging, the role of natural disturbances in defining old growth is more ambiguous. Some definitions, for example, exclude recently burned forests, even where fire has been part of the natural dynamics for millennia; in other cases, such natural disturbance is incorporated in the old growth concept. However, it is often difficult to distinguish the ecological effects of natural disruption from human-caused disruption. Finally, even forests that have never experienced direct manipulation by Euro-Americans have been subjected to indirect effects in the form of invasive species, climate change, and regional modifications of ecological disturbance regimes (e.g., fire suppression).