Nanoparticles are of interest in a variety of biomedical applications that take advantage of their small size and unique properties. Iron oxide magnetic nanoparticles are one class of nanomaterials that respond to externally applied magnetic fields due to their superparamagnetism, enabling applications in targeted and externally triggered drug delivery, magnetic actuation of cell surface receptors, nanoscale heat delivery, and imaging applications. In all these applications, the nanoparticles must navigate biological fluids, which are often composed of a complex, crowded, and confined aqueous mixture of biomacromolecules and salts, bounded by cell membranes and other tissue components. Design of magnetic nanoparticles for biomedical applications would benefit from fundamental understanding of their diffusive behavior in biological fluids. However, traditional techniques to characterize colloid diffusion often fail in biological environments due to the presence of nanoscale biomolecules, cell debris, and cells. In this talk I will summarize our work on evaluating diffusion of nanoparticles in polymer solutions and biological fluids through dynamic magnetic susceptibility (DMS) measurements and x-ray photocorrelation spectroscopy (XPCS). DMS measurements take advantage of the balance between magnetic and hydrodynamic torques on magnetically blocked nanoparticles subjected to alternating magnetic fields. The frequency spectrum of the nanoparticle response is analyzed quantitatively to calculate their rotational diffusion coefficient. The measurement relies on magnetic signals, does not require optic access, and provides reliable measurements even complex fluid environments. XPCS measurements analyze the dynamic scattering of x-rays by suspended colloids to calculate their translational diffusion coefficient. Results for diffusion of polymer grafted nanoparticles in polymer solutions, hyaluronic acid solutions, and in synovial fluid will be presented.