This thesis focused on the development and application of optical microscopy-based spectroscopic techniques to investigate phase transitions associated with crystallisation and biomineralisation. In particular, a micro-dynamic light scattering (micro-DLS) setup and a home-built Raman microspectroscope were developed, enabling time-resolved and in situ measurements with high spatial resolution. The main in vitro model system studied throughout the thesis was calcium carbonate, a relevant biomineral. Combined optical microscopy and Raman microspectroscopy revealed the coexistence of multiple phases during precipitation of calcium carbonate over a wide range of supersaturation and identified a previously overlooked bicarbonate-rich gel phase, highlighting the complexity and heterogeneity of the crystallisation process. Further investigations showed that the gel constitutes a distinct phase, structurally and morphologically different from particulate amorphous calcium carbonate (ACC). Finally, Raman microspectroscopy was applied to a biomineralisation case study, allowing the in vivo identification and localisation of barite microcrystals within Spirogyra cells.