One of the fundamental challenges of nanoscience and nanotechnology today is to organize nanoparticles, now increasingly recognized as 'artificial atoms', into higher-ordered structures analogous to molecules, polymers, and crystals. Inorganic nanoparticles in particular have garnered significant interest for their unique size- and morphology-dependent properties in the nanoscale, much unlike bulk materials which typically possess constant physical properties. Furthermore, the collective properties of nanoparticle assemblies in well-defined spatial configurations and crystal lattices can result in a dramatically different electromagnetic coupling profile among nanoparticles as compared to those of materials in the bulk phase, isolated nanoparticles, and disordered nanoparticle assemblies. The potential for a new generation of designer materials and devices created from nanoparticle "atoms" spurred on developments in the nanoparticle assembly that included the rapid rise of DNA-based nanoparticle assembly. DNA has long been investigated as genetic material but has quickly emerged in the past decade as excellent "generic" polymer that can be potentially engineered to organize complex, functional systems through self-assembly. Aside from its high monodispersity and outstanding stability, DNA possess the unique advantage of programmability via Watson-Crick basepairing. In the last decade alone, DNA-based nanoparticle "molecules", "polymers" and "crystals" have been successfully demonstrated in a variety of formats, configurations and with different materials. In spite of the recent suc- cesses, there is still a tremendous lack of understanding on the nanoscale interactions that govern the assembly process, thus limiting the customizability, complexity, and utility of the final assembled product. The works discussed in this dissertation not only seek to challenge existing notions of DNA-based assembly but also to provide new insights into DNA-based nanoparticle assembly that could pave the way towards a blueprint for rational design of complex nanoparticle assemblies. In the following chapters, we explore how nanoparticles capped with linear DNA strands assemble into tunable crystals in various formats by rational control over DNA length, DNA sequence and ionic environment. In addition, we present how branched DNA can also be rationally designed to direct the assembly of nanoparticle molecules.