Kelp forests, characterized by their high biodiversity, structural complexity, and productivity, are found in cold-water rocky marine coastlines. This review examines the global conditions under which kelp forests develop, the factors leading to their deforestation, and the long-term archaeological history of these ecosystems. Kelp forests are physiologically constrained by light at high latitudes and by nutrients, warm temperatures, and other macrophytes at low latitudes. In mid-latitude regions (40-60° latitude), herbivory, primarily from sea urchins, is the primary threat to kelp forests. Overfishing and the extirpation of apex predators, such as large predatory fishes, often trigger herbivore population increases, leading to widespread kelp deforestation. The impacts of such deforestation are particularly profound in species-depleted systems, such as those in Alaska and the western North Atlantic. Over the past two decades, urchin-induced deforestation has increased globally, and continued fishing down of coastal food webs has shifted harvesting targets from apex predators to their invertebrate prey, including kelp-grazing herbivores. Recent global expansion of sea urchin harvesting has led to the widespread extirpation of this herbivore, and kelp forests have returned in some locations, but these forests now lack vertebrate apex predators. In the western North Atlantic, large predatory crabs have filled the void left by the extirpation of predatory fishes, becoming the new apex predators. Similar shifts from fish- to crab-dominance may have occurred in coastal zones of the United Kingdom and Japan, where large predatory finfish were extirpated long ago. The review also examines three North American case studies—Alaska, the western North Atlantic, and southern California—to determine their long history with humans and project the status of future kelp forests to the year 2025. Fishing impacts on kelp forest systems have been profound and longer-lasting than previously thought. Archaeological data suggest that coastal peoples exploited kelp forest organisms for thousands of years, occasionally resulting in localized losses of apex predators and outbreaks of sea urchin populations. Over the past two centuries, commercial exploitation for export led to the extirpation of sea urchin predators and predatory fishes, increasing sea urchin abundances and promoting the decline of kelp forests. In southern California, widespread deforestation is rare, possibly due to functional redundancies among predators and herbivores, which make this system more stable and resistant to invasion by non-native species. In the western North Atlantic, introduced algal competitors carpet the benthos and threaten future kelp dominance. Climate change has also had measurable impacts on kelp forest ecosystems, and efforts to control greenhouse gas emissions should be a global priority. However, overfishing remains the greatest manageable threat to kelp forest ecosystems over the next two decades. Management should focus on minimizing fishing impacts and restoring populations ofKelp forests, characterized by their high biodiversity, structural complexity, and productivity, are found in cold-water rocky marine coastlines. This review examines the global conditions under which kelp forests develop, the factors leading to their deforestation, and the long-term archaeological history of these ecosystems. Kelp forests are physiologically constrained by light at high latitudes and by nutrients, warm temperatures, and other macrophytes at low latitudes. In mid-latitude regions (40-60° latitude), herbivory, primarily from sea urchins, is the primary threat to kelp forests. Overfishing and the extirpation of apex predators, such as large predatory fishes, often trigger herbivore population increases, leading to widespread kelp deforestation. The impacts of such deforestation are particularly profound in species-depleted systems, such as those in Alaska and the western North Atlantic. Over the past two decades, urchin-induced deforestation has increased globally, and continued fishing down of coastal food webs has shifted harvesting targets from apex predators to their invertebrate prey, including kelp-grazing herbivores. Recent global expansion of sea urchin harvesting has led to the widespread extirpation of this herbivore, and kelp forests have returned in some locations, but these forests now lack vertebrate apex predators. In the western North Atlantic, large predatory crabs have filled the void left by the extirpation of predatory fishes, becoming the new apex predators. Similar shifts from fish- to crab-dominance may have occurred in coastal zones of the United Kingdom and Japan, where large predatory finfish were extirpated long ago. The review also examines three North American case studies—Alaska, the western North Atlantic, and southern California—to determine their long history with humans and project the status of future kelp forests to the year 2025. Fishing impacts on kelp forest systems have been profound and longer-lasting than previously thought. Archaeological data suggest that coastal peoples exploited kelp forest organisms for thousands of years, occasionally resulting in localized losses of apex predators and outbreaks of sea urchin populations. Over the past two centuries, commercial exploitation for export led to the extirpation of sea urchin predators and predatory fishes, increasing sea urchin abundances and promoting the decline of kelp forests. In southern California, widespread deforestation is rare, possibly due to functional redundancies among predators and herbivores, which make this system more stable and resistant to invasion by non-native species. In the western North Atlantic, introduced algal competitors carpet the benthos and threaten future kelp dominance. Climate change has also had measurable impacts on kelp forest ecosystems, and efforts to control greenhouse gas emissions should be a global priority. However, overfishing remains the greatest manageable threat to kelp forest ecosystems over the next two decades. Management should focus on minimizing fishing impacts and restoring populations of