An old-growth forest — termed primary forest, virgin forest, primeval forest, late seral forest, or forest primeval — is a forest that has attained great age without significant disturbance and thereby exhibits unique ecological features and might be classified as a climax community. Old-growth features include diverse tree-related structures that provide diverse wildlife habitat that increases the biodiversity of the forested ecosystem; the concept of diverse tree structure includes multi-layered canopies and canopy gaps varying tree heights and diameters, diverse tree species and classes and sizes of woody debris. Old-growth forests are valuable for economic reasons and for the ecosystem services they provide; this can be a point of contention when some in the logging industry desire to cut down the forests to obtain valuable timber, while environmentalists seek to preserve the forests for benefits such as maintenance of biodiversity, water purification, flood control, nutrient cycling. Proforestation — the practice of purposefully growing an existing forest intact toward its full ecological potential — aims to promote old-growth forests.

Old-growth forests tend to have large trees and standing dead trees, multilayered canopies with gaps that result from the deaths of individual trees, coarse woody debris on the forest floor. Forest regenerated after a severe disturbance, such as wildfire, insect infestation, or harvesting, is called second-growth or'regeneration' until enough time passes for the effects of the disturbance to be no longer evident. Depending on the forest, this may take from a century to several millennia. Hardwood forests of the eastern United States can develop old-growth characteristics in 150–500 years. In British Columbia, old growth is defined as 120 to 140 years of age in the interior of the province where fire is a frequent and natural occurrence. In British Columbia’s coastal rainforests, old growth is defined as trees more than 250 years, with some trees reaching more than 1,000 years of age. In Australia, eucalypt trees exceed 350 years of age due to frequent fire disturbance. Forest types have different development patterns, natural disturbances and appearances.

A Douglas-fir stand may grow for centuries without disturbance while an old-growth ponderosa pine forest requires frequent surface fires to reduce the shade-tolerant species and regenerate the canopy species. In the Boreal-West Forest Region, catastrophic disturbances like wildfires minimize opportunities for major accumulations of dead and downed woody material and other structural legacies associated with old growth conditions. Typical characteristics of old-growth forest include presence of older trees, minimal signs of human disturbance, mixed-age stands, presence of canopy openings due to tree falls, pit-and-mound topography, down wood in various stages of decay, standing snags, multilayered canopies, intact soils, a healthy fungal ecosystem, presence of indicator species. Old-growth forests are biologically diverse, home to many rare species, threatened species, 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, geographic variables. Logging in old-growth forests is a contentious issue in many parts of the world. Excessive logging reduces biodiversity, affecting not only the old-growth forest itself, but indigenous species that rely upon old-growth forest habitat. A forest in old-growth stage has a mix of tree ages, due to a distinct regeneration pattern for this stage. New trees regenerate at different times from each other, because each of them has different spatial location relative to the main canopy, hence each one receives a different amount of light; the mixed age of the forest is an important criterion in ensuring that the forest is a stable ecosystem in the long term. A climax stand, uniformly aged becomes senescent and degrades within a short time to result in a new cycle of forest succession. Thus, uniformly aged stands are less stable ecosystems.

Forest canopy gaps are essential in maintaining mixed-age stands. Some herbaceous plants only become established in canopy openings, but persist beneath an understory. Openings are a result of tree death due to small impact disturbances such as wind, low-intensity fires, tree diseases. Old-growth forests are unique having multiple horizontal layers of vegetation representing a variety of tree species, age classes, sizes, as well as "pit and mound" soil shape with well-established fungal nets; because old-growth forest is structurally diverse, it provides higher-diversity habitat than forests in other stages. Thus, sometimes higher biological diversity can be sustained in old-growth forest, or at least a biodiversity, different from other forest stages; the characteristic topography of much old-growth forest consists of mounds. Mounds are caused by decaying fallen trees, pits by the roots pulled out of the ground when trees fall due to natural causes, including being pushed over by animals. Pits expose humus-poor, mineral-rich soil and collect moisture and fallen leaves, forming a thick organic layer, able to nurture certain types of organisms.

Mounds provide a place free of leaf inundation and saturation, where other types of organisms thrive. Standing snags provide food sources and habitat for many types of organisms. In particular, many species of dead-wood predators such as woodpeckers must have standing snags available for feeding. In North America, the spotted owl is well kn

Sarah Jayne Bell is a Professor of Environmental Engineering based at The Bartlett School in University College London. She is Director of the UCL Engineering Exchange. Bell studied chemistry and environmental engineering at the University of Western Australia and graduated with a Bachelor's of Science and Bachelor's of Engineering in 1996, she moved to the University of New England in Australia for her graduate studies and earned her master's degree in environmental management in 1999. Bell was a doctoral student at Murdoch University, where she worked on sustainability and technology policy and completed her PhD in 2004. In 2005 Bell joined University College London. After moving to the United Kingdom, Bell was awarded an Engineering and Physical Sciences Research Council Research Fellowship on Living With Environmental Change, her research considers urban water systems and infrastructure provision, involves collaboration between engineers and their local communities. Bell has studied the ability of health systems to respond to climate change.

She has worked with Thames Water and Arup Group. She was promoted to Professor in September 2018. Bell identified that there was not much collaboration between engineers and local communities. In an effort to mitigate this lack of communication, Bell founded the University College London Engineering Exchange, she worked with UCL Urban Laboratory to launch a review into social housing, which identified that demolition decisions are made by professional bodies without adequate engagement with residents. She is part of the Community Water Management for a Liveable London, which looks to improve decision making through community and industry engagement; as part of CAMELLIA Bell has looked to make London's water supply more sustainable. Bell is committed to teaching and her efforts have been recognised by the Royal Academy of Engineering and University College London, she is a Fellow of the Chartered Institution of Water and Environmental Management and Institution of Civil Engineers. Her publications include: Bell, Sarah.

Urban Water Sustainability: Constructing Infrastructure for Cities and Nature. Routledge. ISBN 9781138929906. Bell, Sarah. "Assessment of building-integrated green technologies: A review and case study on applications of Multi-Criteria Decision Making method". Sustainable Cities and Society. 27: 106–115. Doi:10.1016/j.scs.2016.06.013. Hdl:2086/17084. Bell, Sarah. "Renegotiating urban water". Progress in Planning. 96: 1–28. Doi:10.1016/j.progress.2013.09.001

Valeria Morales Delgado is a Colombian model and beauty pageant titleholder, crowned Miss Universe Colombia 2018 on 30 September 2018. She represented Colombia at the Miss Universe 2018 pageant in Bangkok, Thailand but failed to place in the Top 20. On January 2, 1998, Morales grew up in Cali, she is a Social Communication student from Broward College, United States. She is the owner of Professional Models Fashion Academy. Morales was crowned as Miss Universe Colombia 2018 held on September 30, 2018 at the Club El Rodeo in Medellín, Colombia, she succeeded outgoing Miss Universe Colombia 2017 and Miss Universe 2017's 1st Runner-up Laura González. As Miss Universe Colombia 2018, Morales represented Colombia at the Miss Universe 2018 pageant in Bangkok, Thailand. Although considered a favorite and front runner, Morales failed to place in the Top 20, ending Colombia’s four year streak of consecutive placements, from 2014 through 2017. Official Miss Colombia website