A materials synthesis technique was recently developed to generate polymer/nanoparticles composite microcapsules in which synthetic polyamines such as polyallylamine and/or polylysine were crosslinked with multivalent anions to form polymer-salt aggregates, that then served as templates for deposition of nanoparticles (NPs) of various compositions to form micron-sized hollow spheres or "nanoparticle-assembled capsules" (NACs). This electrostatically-driven "polymer-salt aggregate" or "PSA" assembly route is attractive for encapsulation and scale-up because encapsulation and materials formation occur in water, at mild pH values, and at room temperature. NACs can potentially find wide-ranging applications in pharmaceutical, food, and consumer products. It is of crucial importance to address the physical property aspects of NACs in view of their use and applicability. While most applications may require that NACs not disassemble or deform under shear stress, some may require triggered release under specific conditions to release the encapsulated material (e.g., enzymes or drugs). Comparatively, little has been done to assess the physical properties of NACs. The behavior of NACs under varying p1-1 and ionic strength conditions were determined. The capsules were found to be structural intact in the pH range of 4-9 at an ionic strength of 10 mM. The pH range in which they were intact narrowed with increasing ionic strength; the capsules fragmented into smaller pieces at 500 mM. The NACs could be made stable at ionic strengths as high as 1M by the addition of multivalent anions to the suspending fluid. The structurally intact NACs were found to vary in compressive strength from 1 atm to > ∼25 atm, via osmotic pressure studies. The benign assembly conditions of NACs allowed for encapsulation studies of various molecules such as fluorescein, Gd[DOTP] 5- (MRI contrast agent), doxorubicin (an anticancer drug), and uracil (pharmaceutical drug with anticancer properties). X-ray irradiation was studied as a potential external trigger for cargo release. A thorough experimental analysis on diffusive release of a dye molecule (fluorescein) from NACs was carried out. Manipulation of the PSA assembly process was carried out in several studies to explore the generality of the synthesis method. Positively-charged aluminosilicate NPs were studied in place of negatively-charged silica NPs. Surprisingly, these led to solid microspheres instead of hollow microspheres. Following the diffusion-deposition model for microsphere formation, it is seems that the NPs, with positively charged alumina patches on top of a negatively charged silica surface, can fully penetrate into the polymer-salt aggregate to form the solid microspheres. The viscoelastic nature of polymer-salt aggregates was exploited to produce non-sphere-shaped NACs through the use of a high-shear flow instrument (Reynolds number of ∼21,000). A mathematical model was developed to understand the formation of elongated NACs, which indicated the shear and elongational stresses within the boundary layer zones along the flow channel walls were responsible for the observed formation of rod-like microparticles.