In this feature article, syntheses of nanosized particles by using colloidal assemblies as a template are described. We asked ourselves the following question : What parameters play a role in the control of the size, shape, and polydispersity? We know that parameters such as the shape of colloidal assemblies, the hydration of the head polar group, the water molecules bounded to the interface, etc. play a major role. However, there are a number of exceptions preventing any generalization. It is shown that the chemical mechanism in nanoparticles production in colloidal assemblies can differ from those usually observed in homogeneous solution. This shows that the solution chemistry cannot always be transferred to colloidal systems. It is possible to select the size and markedly reduce the polydispersity by a surface treatment of the nanoparticles. This favors formation of mono- and multilayers made of nanoparticles, and it is found that these particles form crystals organized in a three-dimensional face-centered cubic superlattice.