This review examines the relationship between depression and dementia, focusing on the timing of depression and its potential mechanisms. Early-life depression, defined as depression occurring before age 60, has been consistently associated with a 2-fold or greater increase in the risk of dementia. In contrast, studies on late-life depression have shown conflicting results, with most supporting an association but varying in their conclusions. The mechanisms linking depression to dementia include vascular disease, alterations in glucocorticoid steroids and hippocampal atrophy, increased deposition of β-amyloid plaques, inflammatory changes, and deficits of nerve growth factors. Treatment strategies for depression may intervene on these pathways and potentially alter the risk of dementia. Given the projected increase in dementia cases, understanding whether treating depression alone or in combination with other interventions can improve cognition is crucial. The review highlights the need for further research to clarify the nature of the association between depression and dementia, particularly in late-life depression, and to explore potential mechanisms for targeted interventions.This review examines the relationship between depression and dementia, focusing on the timing of depression and its potential mechanisms. Early-life depression, defined as depression occurring before age 60, has been consistently associated with a 2-fold or greater increase in the risk of dementia. In contrast, studies on late-life depression have shown conflicting results, with most supporting an association but varying in their conclusions. The mechanisms linking depression to dementia include vascular disease, alterations in glucocorticoid steroids and hippocampal atrophy, increased deposition of β-amyloid plaques, inflammatory changes, and deficits of nerve growth factors. Treatment strategies for depression may intervene on these pathways and potentially alter the risk of dementia. Given the projected increase in dementia cases, understanding whether treating depression alone or in combination with other interventions can improve cognition is crucial. The review highlights the need for further research to clarify the nature of the association between depression and dementia, particularly in late-life depression, and to explore potential mechanisms for targeted interventions.