Knowledge about the mechanisms involved in the structural development of solid materials at the atomic level is essential for designing rational synthesis protocols for these compounds, which may be used to improve desired technical properties, such as light emission, conductivity, magnetism, porosity or particle size, and may allow the tailored design of solid materials to generate the aforementioned properties. Recent technological advancements have allowed the combination of synchrotron-based in situ X-ray diffraction (XRD) with in situ optical spectroscopy techniques, providing researchers with remarkable opportunities to directly investigate structural changes during synthesis reactions. Among the various available methods to measure optical properties, in situ luminescence, UV/Vis absorption, and light transmission spectroscopies are highlighted here, with in situ luminescence being subdivided into in situ luminescence analysis of coordination sensors (ILACS) and time-resolved laser fluorescence spectroscopy (TRLFS). This article consists of a review of 122 references exploring various aspects of in situ analyses, with particular emphasis on the use of XRD-combined techniques in the study of metal-ligand exchange processes during the formation, phase transitions and decomposition of solid materials, including complexes, coordination polymers, metal-organic frameworks, nanoparticles and polyoxo- or chalcogenide metallates. We will then conclude with an exploration of future trends in this exciting research field.