Abstract: Perovskite solar cells (PSCs) are promising next-generation photovoltaics due to their unique optoelectronic properties and rapid rise in power conversion efficiency. However, the instability of perovskite materials and devices is a serious obstacle hindering technology commercialization. The quality of perovskite films, which is an important prerequisite for long-term stable PSCs, is determined by the quality of the precursor solution and the post-deposition treatment performed after perovskite formation. Herein, we review the origin of instability of solution-processed PSCs from the perspectives of the precursor solutions and the perovskite films. In addition, we summarize the recent strategies for improving the stability of the perovskite films. Finally, we pinpoint possible approaches to further advance their long-term stability.