<p>Time-resolved small-angle X-ray scattering (TR-SAXS) is an indispensable technique for directly monitoring in situ kinetic processes in soft matter, providing real-time structural information from nanometer to micrometer length scales. This Focus Review summarizes recent significant advances in understanding the self-assembly kinetics of block copolymers in solution, primarily revealed through TR-SAXS. The review is structured into three key sections, each addressing distinct driving forces and mechanisms. First, we discuss the formation and transformation of micelles, predominantly driven by non-covalent interactions like van der Waals forces and hydrophobic interactions, leading to the spontaneous association of amphiphilic block copolymers in selective solvents. Next, we cover polyelectrolyte complex micelles and vesicles, where self-assembly is initiated by electrostatic interactions, as mixing oppositely charged block polyelectrolytes in aqueous media forms complex coacervate structures. Finally, we present polymerization-induced self-assembly (PISA), a unique approach involving the in situ formation and evolution of block copolymer nanostructures as a monomer is polymerized from a pre-existing polymer chain, simultaneously achieving block copolymer synthesis and self-assembly. Through these examples, we highlight the power of TR-SAXS in elucidating the intricate kinetic pathways and underlying mechanisms governing block copolymer self-assembly.</p>