The unpredictability of scyphozoan jellyfish blooms has been a topic of scientific interest for decades, despite significant research efforts. While factors such as climate change, eutrophication, overfishing, and habitat modification have been proposed as contributors to jellyfish blooms, there is limited robust evidence to support these claims. The belief that jellyfish blooms are increasing in number has been challenged, and while the number of species with at least one recorded bloom has increased in the last decade, the occurrence of blooms remains unpredictable, with years of unexplained absence. The study focuses on the current state of knowledge, uncertainties, and gaps in the critical points that influence the intensity of blooms or their absence. These include factors such as natural and predatory mortality of larvae, the molecular pathway of strobilation, benthic population dynamics, and the success of planula settlement and ephyra to medusa transition. Some of these factors have been studied in controlled conditions, but are difficult to study in the field. The review highlights the need to address different sources of variability to improve understanding of jellyfish population dynamics. Jellyfish blooms have significant socio-economic impacts, including reduced fish catches, economic losses in tourism, and increased costs for beach visitors to avoid jellyfish. However, jellyfish also provide ecosystem services, such as maintaining water quality through top-down control of the food web and being used as food in aquaculture. Additionally, jellyfish have potential applications in pharmacology and medicine, and their biomechanics are studied for the development of remotely operated vehicles. Despite the conceptual framework linking ocean degradation to jellyfish blooms, there are limitations, including a lack of reliable baseline data and short time-series. The classification of jellyfish into a single group of 'gelatinous zooplankton' has been criticized due to their taxonomic, genetic, and life cycle differences. Research efforts have been unevenly distributed, with an overrepresentation of certain species. The class Scyphozoa is particularly important, as it contains many bloom-forming species and is responsible for most of the disturbances in fisheries and aquaculture. Temperature is a key variable in the conceptual framework, influencing the life cycle of scyphozoan jellyfish. However, the effect of temperature can be blurred when considering longer time series. The unpredictability of jellyfish blooms remains a challenge, as the ability to predict them is limited, and the factors influencing their occurrence are complex. The review emphasizes the need to address sources of variability that affect the viability of scyphozoan blooms, including predation on larval stages, parasitism, and interspecific competition. It also highlights the importance of studying factors that can induce physiological stress when not optimal, such as temperature, food availability, or salinity. The review also discusses the life cycle of scyphozoan jellyfish, including the planula, polyp, strobila, and post-strobila stages.The unpredictability of scyphozoan jellyfish blooms has been a topic of scientific interest for decades, despite significant research efforts. While factors such as climate change, eutrophication, overfishing, and habitat modification have been proposed as contributors to jellyfish blooms, there is limited robust evidence to support these claims. The belief that jellyfish blooms are increasing in number has been challenged, and while the number of species with at least one recorded bloom has increased in the last decade, the occurrence of blooms remains unpredictable, with years of unexplained absence. The study focuses on the current state of knowledge, uncertainties, and gaps in the critical points that influence the intensity of blooms or their absence. These include factors such as natural and predatory mortality of larvae, the molecular pathway of strobilation, benthic population dynamics, and the success of planula settlement and ephyra to medusa transition. Some of these factors have been studied in controlled conditions, but are difficult to study in the field. The review highlights the need to address different sources of variability to improve understanding of jellyfish population dynamics. Jellyfish blooms have significant socio-economic impacts, including reduced fish catches, economic losses in tourism, and increased costs for beach visitors to avoid jellyfish. However, jellyfish also provide ecosystem services, such as maintaining water quality through top-down control of the food web and being used as food in aquaculture. Additionally, jellyfish have potential applications in pharmacology and medicine, and their biomechanics are studied for the development of remotely operated vehicles. Despite the conceptual framework linking ocean degradation to jellyfish blooms, there are limitations, including a lack of reliable baseline data and short time-series. The classification of jellyfish into a single group of 'gelatinous zooplankton' has been criticized due to their taxonomic, genetic, and life cycle differences. Research efforts have been unevenly distributed, with an overrepresentation of certain species. The class Scyphozoa is particularly important, as it contains many bloom-forming species and is responsible for most of the disturbances in fisheries and aquaculture. Temperature is a key variable in the conceptual framework, influencing the life cycle of scyphozoan jellyfish. However, the effect of temperature can be blurred when considering longer time series. The unpredictability of jellyfish blooms remains a challenge, as the ability to predict them is limited, and the factors influencing their occurrence are complex. The review emphasizes the need to address sources of variability that affect the viability of scyphozoan blooms, including predation on larval stages, parasitism, and interspecific competition. It also highlights the importance of studying factors that can induce physiological stress when not optimal, such as temperature, food availability, or salinity. The review also discusses the life cycle of scyphozoan jellyfish, including the planula, polyp, strobila, and post-strobila stages.