Solution-phase synthesis of colloidal nanoparticles with precisely tailored properties is one of the fastest growing research topic and represents the most critical foundation to implant nanotechnology in a variety of areas to boost performance of traditional systems. Comprehensive understanding of the nucleation and growth mechanisms involved in the formation of colloidal nanoparticles is very important to realize rational design and synthesis of well-tailored nanoparticles and requires appropriate in situ techniques to probe the kinetics of the synthetic reactions. Synchrotron hard X-rays represent a class of promising probes for solution-phase reactions due to their strong penetration in ambient environment and solutions. This review completely summarizes the in situ synchrotron X-ray techniques emerged in the recent years for real-time probing nanophase evolution of colloidal nanoparticles. Typical examples of colloidal nanoparticle syntheses are discussed in detail to shed the light on the advantages and disadvantages of individual techniques.