Nanoparticles can be used as building blocks of materials. Properties of such materials depend on the organization of the constituent particles. Thus, control over particle organization enables control over material properties. However, robust and scalable methods for arranging nanoparticles are still lacking. This dissertation explores the use of an externally applied magnetic field to organize magnetic nanoparticles into microstructures of desired shape. It extends to proofs of concept towards applications in material design and tissue engineering. First, external control over dipolar self-assembly of magnetic nanoparticles (MNPs) in a liquid dispersion is investigated experimentally. Scaling laws are derived to explain experimental observations, correlating process control variables to microstructure morphology. Implications of morphology on magnetic properties of such structures are then explored computationally. Specifically, a method is proposed wherein superparamangetic nanoparticles, having no residual magnetization, can be organized into anisotropic structures with remanence. Another application explores the use of magnetic forces in organizing human cells into three-dimensional (3D) structures of desired shape and size. When magnetized cells are held in place for several days, they are seen to form inter-cellular contacts and organize themselves into tight clusters. This provides a method for 3D tissue culture without the use of artificial scaffolding materials. Finally, a method to pattern heterogeneities in the stiffness of an elastomer is developed. This makes use of selective inhibition of the catalyst of crosslinking reactions by magnetite nanoparticles. The last chapter discusses future possibilities.