Crystallization is a fundamental process that underlies many natural and industrial processes, from biomineralization and ice formation to the production of pharmaceuticals, manufacturing materials, and food products. Understanding how crystallization occurs and progresses is crucial for advancing research in biomineralization, conservation, environmental and materials sciences. The early stages of crystallization, particularly how atoms coalesce to form a stable nucleus that can grow into a larger crystal, have always been of significant interest. Traditionally, crystallization has been described by the classical nucleation and growth theory (CNT) where first a nucleus of a defined critical size nucleates from solution followed by its growth from monomers addition until the final crystal. However, in the past decade, this theory has been challenged by the emergence of the socalled non-classical crystallization pathway (NCP) due to the observation of stable solute transient species that form in solution before nucleation. In the context of biomineralization, such PNS have been reported for calcium phosphates, carbonates and oxalates. The characterization of these PNS is particularly challenging due to their solubility, nanometric size, dynamic nature, and short lifespan. Numerous analytical approaches have been used to characterize these species, as cryo-TEM approaches, SAXS, monitoring the evolution of calcium potentiometry (for calcium-containing biominerals), DLS, ultracentrifugation, computational studies and by electron ionization mass spectrometry. Despite the number of methodologies used to study these species, there is still a lack of information available in terms of composition, structural arrangement, and kinetic evolution, necessitating the development of innovative time-resolved monitoring techniques.