Sedentary behavior (SB) is a significant factor affecting brain health across the lifespan. While physical activity (PA) is well-documented for its benefits on brain health, the role of SB remains less understood. This review summarizes evidence on SB's impact on cognitive performance, brain structure, and dementia risk across different age groups. It critically evaluates methods to assess SB and explores opportunities for using digital technologies. SB can be categorized as cognitively active (e.g., reading) or passive (e.g., watching TV), with different effects on brain health. For children and adolescents, prolonged SB is linked to poorer cognitive performance, while cognitively active SB may have some benefits. In younger adults, SB is associated with reduced inhibitory control and working memory. In middle-aged and older adults, SB is linked to lower cognitive performance and increased dementia risk, though results are mixed. Observational studies show SB is associated with lower brain-derived neurotrophic factor (BDNF) levels, reduced brain volume, and altered brain connectivity. Interventional studies suggest that PA breaks may mitigate SB's negative effects on brain health. SB assessment methods include self-reports and device-based measures, each with limitations. Future research should focus on larger, longitudinal studies, standardized assessments, and exploring SB's neurobiological mechanisms. SB is often accompanied by other unhealthy behaviors, which should be considered in future studies. Overall, SB is a critical factor in brain health, and further research is needed to understand its complex relationships with cognitive and brain health.Sedentary behavior (SB) is a significant factor affecting brain health across the lifespan. While physical activity (PA) is well-documented for its benefits on brain health, the role of SB remains less understood. This review summarizes evidence on SB's impact on cognitive performance, brain structure, and dementia risk across different age groups. It critically evaluates methods to assess SB and explores opportunities for using digital technologies. SB can be categorized as cognitively active (e.g., reading) or passive (e.g., watching TV), with different effects on brain health. For children and adolescents, prolonged SB is linked to poorer cognitive performance, while cognitively active SB may have some benefits. In younger adults, SB is associated with reduced inhibitory control and working memory. In middle-aged and older adults, SB is linked to lower cognitive performance and increased dementia risk, though results are mixed. Observational studies show SB is associated with lower brain-derived neurotrophic factor (BDNF) levels, reduced brain volume, and altered brain connectivity. Interventional studies suggest that PA breaks may mitigate SB's negative effects on brain health. SB assessment methods include self-reports and device-based measures, each with limitations. Future research should focus on larger, longitudinal studies, standardized assessments, and exploring SB's neurobiological mechanisms. SB is often accompanied by other unhealthy behaviors, which should be considered in future studies. Overall, SB is a critical factor in brain health, and further research is needed to understand its complex relationships with cognitive and brain health.