Cesium tin iodide (CsSnI3) perovskite is a promising candidate material for the photovoltaic application owing to its more ideal bandgap, higher thermal stability, and reduced toxicity compared with the state-of-the-art lead-based hybrid perovskites. However, CsSnI3 suffers from the issue of extreme sensitivity to the ambient atmosphere and the difficulty in manipulating its defect properties. This makes developing efficient stable CsSnI3-based perovskite solar cells (PSCs) a siginificant challenge. In this regard, we have rationally engineered the microstructure/composition of solution-processed CsSnI3 thin films by employing a variety of approaches. These approaches include: i) tailoring the nucleation and grain growth of CsSnI3; ii) modulating the tin-vacancy concentration in the film; iii) alloying of CsSnI3 with other perovskites; and iv) functionalizing the grain boundaries confocally in CsSnI3 thin films. Extensive materials characterization has been performed to demonstrate the validity of these approaches. A combination of these efforts by us have led to CsSnI3-based perovskites solar cells with much improved efficiency and stability. Guidelines for further enhancing the performance of CsSnI3 based PSCs are also provided.