Neuronal ensembles are groups of neurons that display recurring patterns of coordinated activity, serving as intermediate functional units between individual neurons and brain areas. These ensembles can be activated intrinsically or in response to sensory stimuli and play a causal role in perception and behavior. The review discusses the phenomenology, developmental origin, biophysical mechanisms, and functional roles of ensembles across different brain areas and species, including humans. Ensembles are considered modular units of neural circuits that could provide a mechanistic basis for fundamental brain processes such as neural coding, motor planning, decision-making, learning, and adaptability. The concept of ensembles has roots in historical theories, including Cajal's drawings of cortical circuits, Sherrington's reflex arcs, and Lorente de Nó's reverberating chains. Hebb proposed that recurrently connected neurons could form "assemblies" through synaptic plasticity, while Hopfield introduced the idea of "attractors" as stable activity states in neural circuits. Abeles suggested that synchronous activity in neurons, termed "synfire chains," could propagate through the cortex. Recent advances in multielectrode recordings and calcium imaging have enabled the study of ensembles in the hippocampus and neocortex. In the hippocampus, ensembles are involved in spatial memory and can be reactivated during sleep. In the neocortex, ensembles are involved in perceptual tasks and can be created de novo through correlated activity. Both hippocampal and cortical ensembles are endogenous, stable, and can be recruited by sensory inputs or behavioral tasks. The mechanisms of ensemble generation involve both synaptic plasticity and intrinsic changes in excitability. Developmental programs shape the spatial extent and composition of ensembles, with early ensembles emerging from sparse groups of co-active neurons. These ensembles evolve through the formation of chemical synapses and the maturation of GABAergic assemblies. The function of ensembles is supported by their ability to encode information and support cognitive processes. Despite significant progress, challenges remain in understanding the biological properties of ensembles, including their minimum size, spatial and temporal extent, and the role of plasticity. Further research is needed to explore ensembles in other brain areas and to develop methods for their precise identification and manipulation. The study of ensembles provides a framework for understanding the functional organization of neural circuits and the transition from single neuronal activity to complex brain functions.Neuronal ensembles are groups of neurons that display recurring patterns of coordinated activity, serving as intermediate functional units between individual neurons and brain areas. These ensembles can be activated intrinsically or in response to sensory stimuli and play a causal role in perception and behavior. The review discusses the phenomenology, developmental origin, biophysical mechanisms, and functional roles of ensembles across different brain areas and species, including humans. Ensembles are considered modular units of neural circuits that could provide a mechanistic basis for fundamental brain processes such as neural coding, motor planning, decision-making, learning, and adaptability. The concept of ensembles has roots in historical theories, including Cajal's drawings of cortical circuits, Sherrington's reflex arcs, and Lorente de Nó's reverberating chains. Hebb proposed that recurrently connected neurons could form "assemblies" through synaptic plasticity, while Hopfield introduced the idea of "attractors" as stable activity states in neural circuits. Abeles suggested that synchronous activity in neurons, termed "synfire chains," could propagate through the cortex. Recent advances in multielectrode recordings and calcium imaging have enabled the study of ensembles in the hippocampus and neocortex. In the hippocampus, ensembles are involved in spatial memory and can be reactivated during sleep. In the neocortex, ensembles are involved in perceptual tasks and can be created de novo through correlated activity. Both hippocampal and cortical ensembles are endogenous, stable, and can be recruited by sensory inputs or behavioral tasks. The mechanisms of ensemble generation involve both synaptic plasticity and intrinsic changes in excitability. Developmental programs shape the spatial extent and composition of ensembles, with early ensembles emerging from sparse groups of co-active neurons. These ensembles evolve through the formation of chemical synapses and the maturation of GABAergic assemblies. The function of ensembles is supported by their ability to encode information and support cognitive processes. Despite significant progress, challenges remain in understanding the biological properties of ensembles, including their minimum size, spatial and temporal extent, and the role of plasticity. Further research is needed to explore ensembles in other brain areas and to develop methods for their precise identification and manipulation. The study of ensembles provides a framework for understanding the functional organization of neural circuits and the transition from single neuronal activity to complex brain functions.