Pupil size is a widely used metric of brain state, reflecting arousal and neural activity. It is a low-cost, easily measurable signal that can be monitored in various settings. However, its interpretation remains complex, as it is influenced by multiple neuromodulatory systems, including noradrenergic, serotonergic, and orexin/hypocretinergic pathways. Pupil-linked arousal is a term used to describe changes in pupil size and underlying arousal and brain activity, but its precise definition is unclear. Recent studies suggest that pupil size is influenced by a variety of factors, including sensory processing, decision-making, and environmental uncertainty. The relationship between pupil size and arousal is not always straightforward, as arousal may modulate different aspects of behavior in varying ways. Pupil size is also linked to visual processing, with dilation increasing light intake but reducing visual acuity, and constriction improving acuity but reducing sensitivity. These changes are adaptive, allowing animals to optimize visual performance based on task demands. Pupil size is also a social signal, with larger pupils potentially indicating attraction or relaxation. Neurocomputational models have been used to explore how pupil size dynamics relate to ongoing latent neurocomputational processes, such as uncertainty and reward expectation. These models suggest that different brain regions and systems are involved in processing various types of uncertainty, with distinct neural correlates for different levels of uncertainty. The control of pupil size involves multiple brain systems, including the locus coeruleus, dorsal raphe, and lateral hypothalamus, which are interconnected and influence each other. These systems are involved in regulating arousal and behavior, with different roles for different types of arousal. Future research should focus on understanding the precise mechanisms underlying pupil-linked arousal, including the role of different neuromodulatory systems and the interactions between them. This includes investigating how arousal systems are involved in signaling reward anticipation and how they interact with other cognitive processes, such as emotion regulation. Additionally, the impact of slow internal signals, such as circadian rhythms and nutritive states, on pupil-linked arousal remains an area of interest. Overall, the study of pupil size and its relationship to brain function and behavior is an important area of research with many open questions.Pupil size is a widely used metric of brain state, reflecting arousal and neural activity. It is a low-cost, easily measurable signal that can be monitored in various settings. However, its interpretation remains complex, as it is influenced by multiple neuromodulatory systems, including noradrenergic, serotonergic, and orexin/hypocretinergic pathways. Pupil-linked arousal is a term used to describe changes in pupil size and underlying arousal and brain activity, but its precise definition is unclear. Recent studies suggest that pupil size is influenced by a variety of factors, including sensory processing, decision-making, and environmental uncertainty. The relationship between pupil size and arousal is not always straightforward, as arousal may modulate different aspects of behavior in varying ways. Pupil size is also linked to visual processing, with dilation increasing light intake but reducing visual acuity, and constriction improving acuity but reducing sensitivity. These changes are adaptive, allowing animals to optimize visual performance based on task demands. Pupil size is also a social signal, with larger pupils potentially indicating attraction or relaxation. Neurocomputational models have been used to explore how pupil size dynamics relate to ongoing latent neurocomputational processes, such as uncertainty and reward expectation. These models suggest that different brain regions and systems are involved in processing various types of uncertainty, with distinct neural correlates for different levels of uncertainty. The control of pupil size involves multiple brain systems, including the locus coeruleus, dorsal raphe, and lateral hypothalamus, which are interconnected and influence each other. These systems are involved in regulating arousal and behavior, with different roles for different types of arousal. Future research should focus on understanding the precise mechanisms underlying pupil-linked arousal, including the role of different neuromodulatory systems and the interactions between them. This includes investigating how arousal systems are involved in signaling reward anticipation and how they interact with other cognitive processes, such as emotion regulation. Additionally, the impact of slow internal signals, such as circadian rhythms and nutritive states, on pupil-linked arousal remains an area of interest. Overall, the study of pupil size and its relationship to brain function and behavior is an important area of research with many open questions.