When Au nanoparticles are encapsulated within shells of cross-linked, block copolymer amphiphiles, the structure of the shells is determined by the initial interaction between the amphiphile and the nanoparticle surface. In the case of small nanoparticles, for which particle size is comparable to the dimension of the block copolymer ( ρAu/R g ≈ 1), particles act like solutes that are dissolved within polystyrene-block-poly(acrylic acid) (PS-b-PAA) micelle cores. In the case of larger nanoparticles ( ρAu/R g > 1), PS-b-PAA adsorption is templated by the particle surface, and a concentric core-shell structure is formed. The thickness of this shell can be predicted from theoretical models of polymer adsorption onto highly curved surfaces and controlled by varying the ratio of polymer to available nanoparticle surface area. We anticipate that these rules will illustrate how cross-linked copolymer shells with predetermined thickness can be used to stabilize and functionalize a variety of nanoparticle materials.