The objective of this research is to understand and control the assembly of charged nanoparticles (NPs) and polymers to form functional microcapsules. Microcapsules find extensive applications in pharmaceutical, food, and consumer industry by serving as tiny containers to store, deliver, and/or release substances. Nanoparticle-assembled capsules (NACs) are one such model structure, in which NPs and polymer assemble into stable closed-shell structure. Their highly tunable structural features coupled with the nanoscale properties of NPs lead to exciting prospects in encapsulation and controlled release applications. The formation of NACs involves the charge interactions of polymer and NPs. An early form of NACs involved positively-charged polymers combining with negatively-charged gold NPs to form sub-micron-sized spherical aggregates. Water-filled, hollow microspheres were subsequently obtained upon combination of negatively-charged silica NPs with a suspension of gold NP-polymer aggregates, whereupon the silica NPs formed a thick shell. It was discovered that spherical polymer aggregates could be formed by combining a solution of cationic polymers with a multivalent anion salt solution, increasing the versatility of the NP assembly method. These aggregates, upon shell formation with NPs of silica or other negatively-charged compositions, yielded inorganic/polymer NACs. Aggregate formation was investigated as a function of charge ratio, pH, and time through dynamic light scattering, electrophoretic mobility measurements, chloride ion measurements, and optical microscopy. This two-step synthesis technique is unique among other capsule preparation routes, as it allows the rapid and scalable formation of shells at room temperature, in near-neutral water, and with readily available precursors. These benign synthesis conditions allow encapsulation of sensitive molecules such as enzymes. The synthesis, characterization, and activity of acid phosphatase-containing NACs were studied to demonstrate ease of encapsulation and recoverability. Finally, combining two dissimilar positively-charged polymers and a multivalent anion unexpectedly led to the formation of internally segregated polymer aggregates. These aggregates templated the synthesis of (sub)micron particles with an anisotropic polymer distribution and a surface with discrete polymer patches. The effects of charge and polymer ratios on polymer distribution in these "patchy" particles were studied. Controlled localization of gold NPs within these patches was demonstrated, providing a way to selectively functionalize the surface patches.