The study of jellyfish blooms has gained significant attention due to their impact on human activities, such as fisheries and tourism. Despite extensive scientific research over the past three decades, the occurrence of these blooms remains unpredictable, and their consequences are unavoidable. Climate change, eutrophication, overfishing, coastal construction, and species translocation have been suggested as stressors that increase jellyfish blooms, but robust evidence supporting these claims is limited. The belief that jellyfish blooms are increasing in number has been challenged, and while the class Scyphozoa is responsible for most bloom-forming species, the number of species with recorded blooms has increased in recent years. Long-term time series analysis reveals seasonality in the dynamics of each blooming jellyfish species, but blooms vary in intensity, and there are years of unexplained absence. This review focuses on current knowledge, uncertainties, and gaps in critical points that influence the intensity of blooms or lead to the absence of medusa populations. These points include ephyrae, planulae, and scyphistoma mortality, the molecular pathway of strobilation, benthic population dynamics, planula settlement, and ephyra to medusa transition success. While some of these factors have empirical laboratory evidence, others are difficult to study in the field, and addressing the sources of variability is crucial for improving our understanding of jellyfish population dynamics.The study of jellyfish blooms has gained significant attention due to their impact on human activities, such as fisheries and tourism. Despite extensive scientific research over the past three decades, the occurrence of these blooms remains unpredictable, and their consequences are unavoidable. Climate change, eutrophication, overfishing, coastal construction, and species translocation have been suggested as stressors that increase jellyfish blooms, but robust evidence supporting these claims is limited. The belief that jellyfish blooms are increasing in number has been challenged, and while the class Scyphozoa is responsible for most bloom-forming species, the number of species with recorded blooms has increased in recent years. Long-term time series analysis reveals seasonality in the dynamics of each blooming jellyfish species, but blooms vary in intensity, and there are years of unexplained absence. This review focuses on current knowledge, uncertainties, and gaps in critical points that influence the intensity of blooms or lead to the absence of medusa populations. These points include ephyrae, planulae, and scyphistoma mortality, the molecular pathway of strobilation, benthic population dynamics, planula settlement, and ephyra to medusa transition success. While some of these factors have empirical laboratory evidence, others are difficult to study in the field, and addressing the sources of variability is crucial for improving our understanding of jellyfish population dynamics.