Perovskite solar cell (PSC) is one of the most promising next-generation photovoltaic technologies that can provide low-cost, alternative renewable energy. The main hindrances to the deployment of PSC panels are instability issues. Deterioration in the power conversion efficiency of PSCs over time greatly originates from perovskite light absorbers. Hence, in this thesis, the stability of lead and tin perovskites is investigated. Some key factors to improve the stability of perovskite materials are discussed, including processing, materials, and understanding degradation mechanisms in particular. I introduce a state-of-the-art characterisation technique, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), to investigate the surface stability and degradation process of perovskites. We propose the moisture-induced degradation behaviour of a prototypical halide perovskite, methylammonium lead iodide (MAPI): MAPI decomposes into lead iodide and hydrocarbon chains by releasing hydrogen iodide and ammonia gases. For processing, we find that MAPI films made by aerosol-assisted chemical vapour deposition (AACVD) generally have better stability in humid air than their spin-coated counterparts, which can be attributed to larger grain sizes. Moreover, surface passivation plays a crucial role in the improved stability against moisture.ãThis can be achieved using excess CH3NH3I (to react with a Pb(SCN)2 precursor) or bulky ammonium iodides for MAPI films. Formamidinium (FA)-based mixed-cation mixed-halide perovskites and Cs2SnI6 double perovskite not only have better water resistance but also undergo different degradation routes compared to MAPI. Upon H2O vapour exposure, the FA cation transforms into CH3NH3+ first, whilst the degraded species of Cs2SnI6, CsI, remains at the surface. It will be shown that these insights can pave the way towards stable PSCs.