The results of recent investigations of the mechanochemical synthesis of CaCO3, Cr2O3 and Nb2O5 nanopowders are reported. With all three materials studied, it is shown that the volume fraction of the matrix phase is crucial to the formation of separate, unagglomerated particles. With Cr2O3 and Nb2O5, amorphous particles were formed by mechanochemical reaction and low temperature heat treatment was required for crystallization. It is shown that, as a 'bottom up' process, mechanochemical processing enables the building up nanoparticles through solid-state chemical reaction in a microscopically uniform environment, leading to the formation of nanoparticles with narrow size distributions.