Although aquatic mammals are elusive subjects, long-term studies of cetaceans have revealed remarkable lifehistory traits, including long life spans, bisexual philopatry, prolonged maternal care, and even menopause. Long-term cetacean research, defined here as studies lasting ≥ 10 years, has also helped shape our understanding of large multilevel societies, fission-fusion dynamics, cultural processes, complex sociality, and cognition. Yet relative to their terrestrial counterparts, little is known about many cetacean societies, especially pelagic species; similarly, collection of biological samples (such as blood, feces, urine) from live subjects is rarely possible. Cetaceans have been severely impacted by human activities, from commercial whaling to fisheries bycatch, prey depletion, habitat loss, and chemical and noise pollution. Longitudinal research, defined as measuring the same individuals repeatedly over time, can provide vital information necessary for devising viable solutions for mitigating these impacts and promoting sustainable practices. This review evaluates key findings gleaned from continuous and systematic longitudinal studies of free-living cetaceans. We present examples for each topic, though our condensed review cannot be comprehensive. Given their adaptations to the marine environment, slow life histories, and complex societies, continued investment in long-term research is vital for both understanding and protecting this taxonomic group.