The review discusses the challenges and advancements in all-perovskite tandem solar cells, particularly focusing on the use of mixed tin-lead (Sn-Pb) perovskites. These materials offer a narrow bandgap, making them suitable for tandem configurations with wide-bandgap lead-based perovskites. However, the performance of these tandems has not yet reached its theoretical potential due to several obstacles, including the low quality of Sn-Pb perovskite films, which are often characterized by poor crystallization dynamics and surface imperfections. The review highlights the importance of surface modifications to improve the efficiency and stability of narrow bandgap solar cells. It also addresses the integration of narrow bandgap subcells in all-perovskite tandems, emphasizing the need for reliable and efficient double- and multi-junction configurations. The origins of efficiency losses in Sn-Pb perovskites, such as crystallization, oxidation, mass loss, and ion movement, are discussed in detail. The review outlines various strategies to mitigate these issues, including the use of additives, encapsulation techniques, and advanced characterization methods. Future research directions are suggested, focusing on the characterization and visualization of surface defects and their evolution under different conditions to guide the development of more efficient and stable perovskite solar cell devices.The review discusses the challenges and advancements in all-perovskite tandem solar cells, particularly focusing on the use of mixed tin-lead (Sn-Pb) perovskites. These materials offer a narrow bandgap, making them suitable for tandem configurations with wide-bandgap lead-based perovskites. However, the performance of these tandems has not yet reached its theoretical potential due to several obstacles, including the low quality of Sn-Pb perovskite films, which are often characterized by poor crystallization dynamics and surface imperfections. The review highlights the importance of surface modifications to improve the efficiency and stability of narrow bandgap solar cells. It also addresses the integration of narrow bandgap subcells in all-perovskite tandems, emphasizing the need for reliable and efficient double- and multi-junction configurations. The origins of efficiency losses in Sn-Pb perovskites, such as crystallization, oxidation, mass loss, and ion movement, are discussed in detail. The review outlines various strategies to mitigate these issues, including the use of additives, encapsulation techniques, and advanced characterization methods. Future research directions are suggested, focusing on the characterization and visualization of surface defects and their evolution under different conditions to guide the development of more efficient and stable perovskite solar cell devices.