The article reviews the current understanding of the impact of sedentary behavior (SB) on brain health across different age groups, from children and adolescents to middle-aged and older adults. It highlights the limited evidence on the role of SB compared to physical activity (PA) and discusses the need for more robust research. The review summarizes observational and experimental studies, examining the associations between SB and cognitive performance, structural and functional brain measures, and dementia risk. It also explores the potential mechanisms linking SB to brain health and the challenges in assessing SB. The article emphasizes the importance of considering the type of SB (cognitively active vs. cognitively passive) and its characteristics (frequency, duration, and timing) as moderators. It suggests that cognitively active SB may have beneficial effects on brain health, while cognitively passive SB is associated with poorer cognitive performance. The review also discusses the use of digital technologies for more accurate and ecological assessments of SB and the need for standardized neuropsychological assessments. Finally, it outlines future research priorities, including the use of innovative measurement approaches and the examination of neurobiological mechanisms.The article reviews the current understanding of the impact of sedentary behavior (SB) on brain health across different age groups, from children and adolescents to middle-aged and older adults. It highlights the limited evidence on the role of SB compared to physical activity (PA) and discusses the need for more robust research. The review summarizes observational and experimental studies, examining the associations between SB and cognitive performance, structural and functional brain measures, and dementia risk. It also explores the potential mechanisms linking SB to brain health and the challenges in assessing SB. The article emphasizes the importance of considering the type of SB (cognitively active vs. cognitively passive) and its characteristics (frequency, duration, and timing) as moderators. It suggests that cognitively active SB may have beneficial effects on brain health, while cognitively passive SB is associated with poorer cognitive performance. The review also discusses the use of digital technologies for more accurate and ecological assessments of SB and the need for standardized neuropsychological assessments. Finally, it outlines future research priorities, including the use of innovative measurement approaches and the examination of neurobiological mechanisms.