Multilayer thin films of sequentially adsorbed polyelectrolytes were utilized as nanoreactors for both metallic (Ag) and semiconductor nanoparticles (PbS). Polyelectrolyte multilayers with a controlled content of free carboxylic acid binding groups were fabricated with weak polyelectrolytes via suitable pH adjustments of the processing solutions. These groups were then used to bind various inorganic ions that were subsequently converted into nanoparticles. Spatial control (at the nanoscale) over the growth of the nanoparticles was achieved via the use of multilayer heterostructures that also contain bilayer blocks that are not able to bind inorganic ions. These "nonbinding" bilayers were fabricated from strong polyacids such as poly(styrene sulfonic acid). TEM imaging of the nanoparticles within the multilayer thin films indicates that the polycation and polyanion layers of these films are highly interpenetrated.