A novel general approach to the preparation of metallic nanoparticles is presented, using gold and platinum as examples. The nanoparticles are synthesized inside shell cross-linked siloxane nanocages, which are prepared in three major steps: i) self-assembly of 3-(triethoxysilyl)-propyl hexadecyl carbamate in ethanol generating 2 nm micelles; ii) shell cross-linking of ethoxy headgroups with dimethyldimethoxy silane, retaining the size and shape of the original micelles; iii) deprotection of the inner carbamate groups with TMSI, followed by removal of the remaining tails, obtaining a porous and hollow structure of 2nm (nanocage) internally tethered with propylamine functionalities. The amine groups are used to adsorb AuCl₄^- and PtCl₆^2- from methanolic solutions into the nanocage in a stoichiometric manner. Cyclic voltammetry, XANES, EXAFS and UV-VIS spectroscopy are used to follow the binding and characterize the products. Upon treatment of the amine-bound metal ions with NaBH₄, metal nanoclusters are formed inside the nanocages. This reduction step regenerates the binding sites at the interior of the nanocages, so that a second exposition to fresh solutions of AuCl₄^- or PtCl₆^2- promotes additional adsorption of metal ions. Subsequent treatment with NaBH₄ reduces the metal attached to amine and promotes the growing of the highly unsaturated nanocluster generated in the first reduction step. The size of the nanoparticle inside the nanocage is fine-tuned by controlling the number of adsorption/reduction cycles; additionally, the surrounding sheet prevents any undesirable nanoparticle agglomeration. Results on the application of this methodology to the preparation of bimetallic nanoparticles as well as on the study of their catalytic properties are also described.