The ultimate goal of nanoscale science and technology is to manipulate single atoms, assemble atoms in a controllable way and design nanostructured materials with the desired physical and chemical properties (e.g. catalytic properties). In order to achieve this goal, the nucleation and growth mechanism of nanocrystals (NCs) as well as the relationship between the macroscopic properties and microscopic structures of nanocrystals should be fully understood. In this tutorial review, we firstly summarize the latest research developments and experimental methods to exploit the nucleation and growth process of nanocrystals, then discuss the essence of nanocrystal catalysis, and finally present our personal perspectives on the opportunities and challenges of this promising field.