Nanocomposites hold tremendous potential as ‘designer’ materials with multifunctional, tunable physical properties. We have synthesized and studied two classes of nanocomposite systems –(a) Magnetorhelogical (MR) fluids with uniformly dispersed Fe nanoparticles and (b) Polypyrrole doped with soft ferrite nanoparticles. Static and dynamic magnetic measurements show a variety of phenomena ranging from superparamagnetism to collective spin-flip transitions. A resonant RF method has been used to map the switching and anisotropy fields. Our studies indicate that the rich cooperative magnetism in these systems is governed not only by the particle size distribution but also by the matrix-mediated interactions.