Book excerpt: Perovskite solar cells are the new hope of next-generation photovoltaic concepts for sustainable energy generation. Regarding their favorable optoelectronic properties, bound electron-hole pairs (so-called excitons) play a significant role and are thoroughly investigated utilizing various spectroscopic methods. Moreover, bandgap instabilities caused by segregation effects in mixed perovskites are analyzed in detail using electroreflectance spectroscopy and structural characterization techniques.