Kelp forests are diverse, complex, and highly productive ecosystems found in cold-water rocky coastal areas. This paper reviews the global distribution and development of kelp forests, examining case studies from southern California, the Aleutian Islands, and the western North Atlantic to understand their biodiversity, stability, and resilience. Kelp forests are physiologically constrained by light at high latitudes and by nutrients, warm temperatures, and other macrophytes at low latitudes. Mid-latitude kelp forests are most threatened by herbivory, often from sea urchins. Overfishing and the extirpation of apex predators have led to increased sea urchin populations and widespread kelp deforestation. This has had the most profound impacts on species-depauperate systems, such as those in Alaska and the western North Atlantic. Global urchin-induced deforestation has increased over the past 2–3 decades. Continued fishing down of coastal food webs has shifted harvesting targets from apex predators to their invertebrate prey, including kelp-grazing herbivores. The 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, for the first time, these forests are devoid of vertebrate apex predators. In the western North Atlantic, large predatory crabs have recently filled this void and have become the new apex predator in this system. Similar shifts from fish- to crab-dominance may have occurred in coastal zones of the United Kingdom and Japan. Three North American case studies of kelp forests were examined 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 both profound and much longer in duration 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, outbreaks of sea urchin populations, and probably small-scale deforestation. Over the past two centuries, commercial exploitation for export led to the extirpation of sea urchin predators, such as the sea otter in the North Pacific and predatory fishes like the cod in the North Atlantic. The large-scale removal of predators for export markets increased sea urchin abundances and promoted the decline of kelp forests over vast areas. Despite southern California having one of the longest known associations with coastal kelp forests, widespread deforestation is rare. It is possible that functional redundancies among predators and herbivores make this most diverse system most stable. Such biodiverse kelp forests may also resist invasion from non-native species. In the species-depauperate western North Atlantic, introduced algal competitors carpet the benthos and threaten future kelp dominance. There, other non-native herbivores and predators have become established and dominant components of this system. Climate changes have had measurable impactsKelp forests are diverse, complex, and highly productive ecosystems found in cold-water rocky coastal areas. This paper reviews the global distribution and development of kelp forests, examining case studies from southern California, the Aleutian Islands, and the western North Atlantic to understand their biodiversity, stability, and resilience. Kelp forests are physiologically constrained by light at high latitudes and by nutrients, warm temperatures, and other macrophytes at low latitudes. Mid-latitude kelp forests are most threatened by herbivory, often from sea urchins. Overfishing and the extirpation of apex predators have led to increased sea urchin populations and widespread kelp deforestation. This has had the most profound impacts on species-depauperate systems, such as those in Alaska and the western North Atlantic. Global urchin-induced deforestation has increased over the past 2–3 decades. Continued fishing down of coastal food webs has shifted harvesting targets from apex predators to their invertebrate prey, including kelp-grazing herbivores. The 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, for the first time, these forests are devoid of vertebrate apex predators. In the western North Atlantic, large predatory crabs have recently filled this void and have become the new apex predator in this system. Similar shifts from fish- to crab-dominance may have occurred in coastal zones of the United Kingdom and Japan. Three North American case studies of kelp forests were examined 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 both profound and much longer in duration 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, outbreaks of sea urchin populations, and probably small-scale deforestation. Over the past two centuries, commercial exploitation for export led to the extirpation of sea urchin predators, such as the sea otter in the North Pacific and predatory fishes like the cod in the North Atlantic. The large-scale removal of predators for export markets increased sea urchin abundances and promoted the decline of kelp forests over vast areas. Despite southern California having one of the longest known associations with coastal kelp forests, widespread deforestation is rare. It is possible that functional redundancies among predators and herbivores make this most diverse system most stable. Such biodiverse kelp forests may also resist invasion from non-native species. In the species-depauperate western North Atlantic, introduced algal competitors carpet the benthos and threaten future kelp dominance. There, other non-native herbivores and predators have become established and dominant components of this system. Climate changes have had measurable impacts