Applications of superparamagnetic magnetic ™uids in biomedicine have been expanding over the last decade due to their tremendous potential for tagging and delivery strategies. Their applications in hyperthermia (HT), magnetic resonance imaging (MRI), gene delivery (GD), magnetic drug targeting (MDT), nanomedicine and regenerative medicine have been largely demonstrated. Key aspects of these magnetic ™uids are that they exhibit magnetisation only in an applied magnetic šeld, they must form stable colloidal suspensions for in vivo biomedical applications and they can be directed to a desired site and serve for controlled targeting in clinical applications. Their successful application in healthcare is strongly dependent on their structural characteristics, specišcally size and size distribution, shape, magnetic susceptibility, and surface chemistry. In this chapter, we review the various synthetic routes, and the surface modišcation providing fair colloidal stabilisation, including various coating materials as well as the usual characterisation techniques of these magnetic ™uids.