Neuronal ensembles, defined as groups of neurons displaying recurring patterns of coordinated activity, represent an intermediate functional level between individual neurons and brain areas. Recent advancements in multielectrode recordings and calcium imaging have enabled the identification and characterization of neuronal population activity patterns, revealing that ensembles can be activated intrinsically or in response to sensory stimuli and play a causal role in perception and behavior. This review explores the phenomenology, developmental origin, biophysical and synaptic mechanisms, and potential functional roles of neuronal ensembles across different brain areas and species, including humans. Ensembles are modular units of neural circuits, providing a mechanistic underpinning for fundamental brain processes such as neural coding, motor planning, decision-making, learning, and adaptability. The review highlights the importance of ensemble activity in both the hippocampus and neocortex, discussing their sequential structure, endogenous nature, and functional roles in spatial cognition and perceptual tasks. It also addresses the mechanisms of ensemble generation and recruitment, including the role of intrinsic excitability changes and interneurons. The development of ensembles is discussed, emphasizing the role of early developmental programs in shaping their spatial extent and composition. Finally, the review outlines challenges and open questions in the study of neuronal ensembles, including the need for rigorous methods to detect and characterize ensembles, the importance of understanding their function in human brains, and the exploration of ensembles in other brain areas and species.Neuronal ensembles, defined as groups of neurons displaying recurring patterns of coordinated activity, represent an intermediate functional level between individual neurons and brain areas. Recent advancements in multielectrode recordings and calcium imaging have enabled the identification and characterization of neuronal population activity patterns, revealing that ensembles can be activated intrinsically or in response to sensory stimuli and play a causal role in perception and behavior. This review explores the phenomenology, developmental origin, biophysical and synaptic mechanisms, and potential functional roles of neuronal ensembles across different brain areas and species, including humans. Ensembles are modular units of neural circuits, providing a mechanistic underpinning for fundamental brain processes such as neural coding, motor planning, decision-making, learning, and adaptability. The review highlights the importance of ensemble activity in both the hippocampus and neocortex, discussing their sequential structure, endogenous nature, and functional roles in spatial cognition and perceptual tasks. It also addresses the mechanisms of ensemble generation and recruitment, including the role of intrinsic excitability changes and interneurons. The development of ensembles is discussed, emphasizing the role of early developmental programs in shaping their spatial extent and composition. Finally, the review outlines challenges and open questions in the study of neuronal ensembles, including the need for rigorous methods to detect and characterize ensembles, the importance of understanding their function in human brains, and the exploration of ensembles in other brain areas and species.