Résumé: Colloidal nanocrystals have important potential applications in different technological fields such as solar cells and thermoelectricity, thanks to their reduced fabrication costs and exotic and unique properties. In this thesis, a variety of advanced transmission electron microscopy (TEM) techniques is used to unravel the morphology, crystal structure and chemical composition of state-of-the-art Cu2ZnSnS4 (CZTS) and CsPbBr3 colloidal nanocrystals. Firstly, the difficulties of these TEM observations and the solutions we found will be presented. Then the details of the various TEM observations, such as electron dispersion spectroscopy (EDS), High Resolution Transmission Electron Microscopy (HR-TEM), High Resolution Scanning Transmission Electron Microscopy (HR-STEM) and Nanobeam Precession Electron Diffraction (NPED) will be summarized and compared to synchrotron Small-Angle X-ray Scattering (SAXS) and Wide-Angle X-ray Scattering (WAXS) techniques. For CZTS, different samples extracted at different times of the fabrication have been observed and a growth mechanism is proposed. For CsPbBr3, the focus was put on the determination of the exact structure, cubic or orthorhombic, of the nanocrystals.