Self-assembly of amphiphilic molecules such as surfactants and amphiphilic block copolymers (BCPs), provides an energy-efficient bottom-up approach for controllably creating structures at the mesoscale (2-50 nm) with potential applications in catalysis, next-generation energy production and storage devices, optical metamaterials and bioengineered materials. Biological systems serve as examples of complex materials at mesoscopic length scales that integrate structural and compositional heterogeneities that lead to functions including toughness, optical iridescence and van der Waals adhesion due to large surface area. In this dissertation, I will describe three different approaches for adding structural complexity to synthetic mesoscale structures. Firstly, controlled synthesis and detailed characterization of multicompartment mesoporous silica nanoparticles (multi-MSNs) from surfactant coassembly with sol-gel silica is described. These multi-MSNs consist of a core with cage-like cubic mesoporous network morphology and up to four fingers/branches with hexagonally packed cylindrical mesopores epitaxially emanating from the vertices of the cubic core. These multi-MSNs are mesoscale structural analogues to branched semiconductor nanocrystals. Possible nucleation and growth processes leading to this particle morphology are discussed. Secondly, multicomponent evaporation-induced self-assembly behavior of ligand-stabilized platinum nanoparticles (Pt NPs) with poly(isoprene-block-dimethylaminoethyl methacrylate) block copolymers is discussed. Detailed characterization on Pt NPs revealed sparse ligand coverage. Changing the volume fraction of Pt NPs in BCP-NP composites yielded organic-inorganic hybrids with spherical micellar, wormlike micellar, lamellar and inverse hexagonal mesoscale morphologies. Disassembly of hybrids with spherical, wormlike micellar, and lamellar morphologies generated isolated metal-NP based nanospheres, cylinders and sheets, respectively. Results suggest the existence of powerful design criteria for the formation of metal-based nanostructures from designer blocked macromolecules. Finally, a facile synthesis protocol for hierarchically structured polymeric scaffolds with highly ordered mesopores is introduced. Mixtures of poly(styrene-block-ethylene oxide) BCPs with oligomeric poly(ethylene oxide) additives were dissolved in high boiling point solvents, and bulk films were cast through solvent evaporation. Spinodal decomposition of the BCP/additive mixture resulted in macrostructure formation, with the BCP-rich domains forming ordered mesostructures. Facile washing of the films resulted in the formation of macro/meso-porous three-dimensional polymer scaffolds. Experimental parameters relevant for structure formation including additive molecular weights, solvents and drying temperatures are explored.