The implementation of magnetic detection techniques in clinical practice has long lagged behind developments in society as a whole. Whilst techniques like magnetic resonance imaging and (to a lesser extent) MEG/MCG have been succesfully introduced, use of magnetic techniques for diagnostics and treatment has always been limited to well controlled, shielded environments. In this thesis, we describe the development of a sensitive and specific method to use magnetic nanoparticles for diagnostics and localization, without the need for shielded surroundings. This opens up a world of new applications for magnetic detection, ranging from intraoperative detection of sentinel lymph nodes to tracking and tracing of stem cell therapies, to navigation applications.