Metal-halide perovskites have garnered intensive interest because of their extraordinary properties for photovoltaic applications. In the last years, the maximum obtained efficiency of perovskite absorbers for this application has risen substantially, up to a maximum of 22%, thereby rapidly closing the gap with the efficiency of more traditional photovoltaic materials. The further development of these perovskites for photovoltaic and other opto-electronic applications is, however, hampered by their limited mechanical and chemical stability. For instance, experiments at ambient conditions reveal that the desired black phase of inorganic perovskites tends to transition towards a yellow nonperovskite phase, which is associated with a strong deterioration of the opto-electronic properties. To understand how this unwanted phase transition can be suppressed, it is crucial to acquire microscopic insight into the physical interactions underpinning the stability of these materials. In this project, a combined experimental and theoretical approach will be followed to obtain a nanoscale understanding of the driving force for these phase transitions. This in-depth understanding will enable us to computationally design and afterwards experimentally synthesize perovskites with an exceptional stability and superior opto-electronic performance exceeding the current state of the art.