Colloidal nanoparticles are an exciting class of materials for their ability to act as tunable building blocks for larger scale materials. The application of nanotechnology to magnetic materials depends on the controlled synthesis of uniform nanoparticles with targeted magnetization and magnetic anisotropy. The work presented in this dissertation has two focuses: one, the development of synthetic techniques which allow for the consistent production of uniform nanoparticles and of novel heterostructured nanoparticles with emergent magnetism, and two, the application of these nanoparticles into bulk magnetic and magnetoresistive assemblies. A common issue which plagues the study of nanoparticles is the variability of synthesis from written protocol to laboratory practice or even from batch to batch. A set of techniques are detailed which have been developed to ensure consistent recreation of reaction conditions which allow for separate LaMer ‘burst’ nucleation and growth regimes during synthesis. High-quality, single-phase nanoparticles synthesized using the preceding techniques were further used as seeds in the synthesis of novel heterostructured nanoparticles exhibiting enhanced exchange bias. Collective magnetism was studied in assemblies of ferrimagnetic Fe3O4 and antiferromagnetic CoO nanoparticles. In typical samples consisting only of permanent magnetic nanoparticles, dipolar interactions between the nanoparticles frustrate the orientation of their magnetic moments, often producing a superspin glass state. Conversely, antiferromagnetic nanoparticles, in their dipolar interactions with ferro- or ferrimagnetic nanoparticles, induce a uniaxial magnetic anisotropy and create a superferromagnetic state in the collective magnetism of the nanoparticle assembly Synthetic control over nanoparticle morphology was exploited to engineer nanoparticles for the assembly of granular magnetoresistance devices. Magnetoresistance measurements on a series of pellets of differently sized CoFe2O4 nanoparticles revealed a size threshold beyond which magnetoresistance was greatly diminished. Additionally, the magnetoresistance of CoFe2O4 was found to be superior to that of Fe3O4 despite the latter’s popularity in magnetoresistance research. Magnetoresistance measurements were also performed on mixed nanoparticle films cast from nanoparticle inks. Tuning of the CoFe2O4 to Fe3O4 ratio in the films successfully produced pseudo spin valve magnetoresistance, improving both the magnitude and responsivity of the films’ magnetoresistance.