The growing interest in nanotechnology stems from its promise of developing new materials and devices by taking advantage of unique phenomena emerging at the nanoscale. In this size regime, parameters like size and shape strongly impact the physico-chemical properties of materials and, therefore, their performance in a particular application. In view of this intimate structure-property-function relationship, significant research efforts have been going on to synthesize nanocrystals (NCs) of various materials, with a high degree of control. Over the years, beautiful works have been reported on the preparation of high-quality NCs with well-defined morphologies, mainly using colloidal chemistry as synthetic tool. However, the predictive character of these approaches is scarce, making the NC synthesis essentially empirical. In order to move towards a rational synthesis design, it is crucial to unravel the mechanisms behind the formation of NCs. The chemistry-sensitivity of in-situ X-rays spectroscopy makes it suitable to probe the key stages of the dynamic processes involved during the NCs nucleation and growth at synchrotron facilities. While great progress has been accomplished for noble metal NCs, the current synthetic state of non-noble metal NCs is less developed; nevertheless they are extremely important for a wide array of technological applications. For this class of materials the challenge becomes to perform in-situ experiments in harsher conditions, such as high temperatures and use of organic solvents, than those required for non-noble metals. Hence, the scope of this thesis is to advance the general synthetic chemistry of colloidal NCs by investigating the underlying formation mechanisms and by developing more predictive synthetic approaches. The system of focus are Cu-based NCs, considering their pivotal role in many fields. We start by exploring the high-temperature syntheses of single-crystalline Cu NCs, using a variety of techniques, including X-ray absorption and scattering at synchrotron facilities. We discover that the synthesis of such relatively simple systems proceed via non-classical nucleation pathways, with the formation of pre-nucleation structures occupying a local minimum in the reaction energy landscape. As the next step, we provide unique insights into their role in determining the final reaction product and achieve a superior monodisperisty for Cu NCs of different sizes as well as a shape not previously reported for these NCs. Turning to more complex systems, we report on the synthetic development of novel ternary copper-based transition metal chalcogenides. Firstly, we focus on the synthesis of colloidal Cu3VS4 NCs, which is an intermediate band gap semiconductor. Thanks to the achieved size-control, we could investigate its optical properties which we found to fall in a weak quantum confinement regime. Moving forward, we contribute to define guidelines towards the rational synthesis design of phase-pure colloidal Cu-M-S NCs (M = V, Cr, Mn), by studying their reaction mechanisms. In particular, exploring the precursor chemistry allowed us to define the same conditions whereby these materials can be prepared. Overall, the work discussed in this thesis contribute to advance the general knowledge of NC formation and furthermore it may also inspire the synthesis of new nanomaterials (e.g. based on non-noble metal NCs) with improved or target properties for the desired application.