It is essential to develop cost-effective and efficient approaches to convert solar energy into other energy forms for our daily use in the near future. Solar cell technology is a crucial tool to effectively tackle this challenging task due to its low cost and flexibility in device design. Recently, hybrid inorganic-organic perovskites were identified as promising light absorbers for solar cell technology owing to their superb power conversion efficiency. Following the remarkable success of applications of CH3NH3PbI3 perovskites in photovoltaics, a great focus has been placed on their stability for establishing foundations for the long lifetimes for perovskite solar cells. In this dissertation, I have investigated the effects of different environmental conditions on physical and chemical properties of hybrid inorganic-organic perovskites.To achieve outdoor applications, the influence of operating conditions (ambient conditions, heat and light) on the stability of such perovskites should be comprehensively studied. Thus, the transformation of CH3NH3PbI3 perovskite films were explored under ambient conditions and at elevated temperatures. It was found that the decomposition of CH3NH3PbI3 into other lead compounds occurred under these conditions. The chemical changes in the decomposed films were found to cause a significant decrease in the photovoltaic efficiency of CH3NH3PbI3. It is also important to study the influence of light illumination on the stability of hybrid perovskite. Under light illumination, halide ion segregation occurring in mixed halide lead perovskites (e.g., CH3NH3PbBrxI3–x) gives rise to the concerns regarding the long-term applications. Herein, the impact of light on mixed halide hybrid perovskite films was investigated by exposing such films to continuous laser irradiation. It turns out that under illumination the generated iodide-rich perovskite species further decompose into metallic lead (Pb0). The chemical changes in decomposed films were found to cause an irreversible shift of absorption band edge and a significant change in film morphology.In addition, the structural transformations of hybrid perovskites are facilitated by interactions with polar molecules (H2O and NH3). Exposure to NH3 gas can significantly change the optical and electronic properties of CH3NH3PbI3 perovskites, which can be explored for gas-sensing applications. However, limited understanding of the processes by which perovskites interact with NH3 further hinders the improvement and full use of the unique properties of these materials. Thus, the effects of ammonia on CH3NH3PbI3 were investigated by exposing perovskite films to an NH3 atmosphere. Spectroscopic analyses indicated that ammonium cations (NH4+) replaced the methylammonium cations (CH3NH3+) in the perovskite crystal, thereby resulting in the formation of NH4PbI3. Nonetheless, the introduction of CH3NH2gas to the formed NH4PbI3 converted it back to CH3NH3PbI3, proving the reversibility of these cation exchange reactions. Lastly, this reversible cation-exchange method was employed to tune the optical and structural properties of colloidal perovskite nanocrystals (NCs). The photoluminescence of the 3D CH3NH3PbBr3 NCs changes from green to violet by adding phenethylammonium cation (PEA+). In contrast, the photoluminescence of the 2D OcA2PbBr4 perovskite NCs turns from violet to green upon introducing methylammonium cation (CH3NH3+). These optical changes are attributed to the reversible structural transformation of perovskite NCs between three-dimensional (3D) and two-dimensional (2D) layered crystalline forms through cation exchange. The reversible cation exchange process opens up new possibilities of using post-synthetic cation interaction to flexibly and rationally tune the optical and structural properties of perovskite materials, bringing greater tailored functionalities to these materials.