This study presents a comprehensive single-cell transcriptomic analysis of the aging mouse ovary, focusing on the period from 3 to 9 months of age, a critical stage before follicular exhaustion. The research reveals significant changes in the ovarian transcriptome, particularly in immune cell populations and follicular cells. Key findings include: 1. **Immune Cell Changes**: The proportion of immune cells in the ovary doubles by 9 months of age, with a marked increase in lymphoid cells, including Type 1 NKT cells and Type 17 γδTs. Flow cytometry confirmed these findings, showing increased CD8+ T cells, 'double-negative αβ T cells', MAIT cells, NKT cells, and γδTs, while CD4+ αβ T cells and innate lymphoid cells decreased. 2. **Follicular Cells**: Granulosa cells (GCs) and theca cells (TCs) display upregulation of stress-response, immunogenic, and fibrotic signaling pathways. GCs show increased activation of the mitochondrial dysfunction pathway, proinflammatory stress pathways, and fibrosis pathways. TCs upregulate TGFβ signaling and cell proliferation pathways. 3. **Stroma Changes**: The stroma shows downregulation of collagenase pathways, leading to increased ovarian fibrosis. Fibroblast-like stromal cells exhibit downregulation of hormonal signaling and inflammation, while TCs show upregulation of fibrogenesis regulators. 4. **Oocyte and Luteal Cells**: Oocytes and luteal cells show increased mitochondrial dysfunction and proinflammatory stress pathways, consistent with age-related declines in oocyte quality. 5. **Endothelial and Epithelial Cells**: Vascular endothelial cells show increased DNA damage regulation and upregulation of cellular senescence markers, while epithelial cells show mild age-related changes. The study highlights the importance of understanding these cellular changes to develop interventions that could extend female fertility and mitigate age-related chronic diseases. The findings provide a critical resource for further research on ovarian aging and its underlying mechanisms.This study presents a comprehensive single-cell transcriptomic analysis of the aging mouse ovary, focusing on the period from 3 to 9 months of age, a critical stage before follicular exhaustion. The research reveals significant changes in the ovarian transcriptome, particularly in immune cell populations and follicular cells. Key findings include: 1. **Immune Cell Changes**: The proportion of immune cells in the ovary doubles by 9 months of age, with a marked increase in lymphoid cells, including Type 1 NKT cells and Type 17 γδTs. Flow cytometry confirmed these findings, showing increased CD8+ T cells, 'double-negative αβ T cells', MAIT cells, NKT cells, and γδTs, while CD4+ αβ T cells and innate lymphoid cells decreased. 2. **Follicular Cells**: Granulosa cells (GCs) and theca cells (TCs) display upregulation of stress-response, immunogenic, and fibrotic signaling pathways. GCs show increased activation of the mitochondrial dysfunction pathway, proinflammatory stress pathways, and fibrosis pathways. TCs upregulate TGFβ signaling and cell proliferation pathways. 3. **Stroma Changes**: The stroma shows downregulation of collagenase pathways, leading to increased ovarian fibrosis. Fibroblast-like stromal cells exhibit downregulation of hormonal signaling and inflammation, while TCs show upregulation of fibrogenesis regulators. 4. **Oocyte and Luteal Cells**: Oocytes and luteal cells show increased mitochondrial dysfunction and proinflammatory stress pathways, consistent with age-related declines in oocyte quality. 5. **Endothelial and Epithelial Cells**: Vascular endothelial cells show increased DNA damage regulation and upregulation of cellular senescence markers, while epithelial cells show mild age-related changes. The study highlights the importance of understanding these cellular changes to develop interventions that could extend female fertility and mitigate age-related chronic diseases. The findings provide a critical resource for further research on ovarian aging and its underlying mechanisms.