It is interesting to imagine what would happen to a bar magnet if it were broken up into smaller and smaller pieces. Eventually one would end up with very small bar magnets, yet their properties could be vastly different from those of large-scale permanent magnets. Even if a macroscopic object made of a ferromagnetic material such as iron or cobalt may have no magnetic polarization, its nanometer-scale counterparts can be made into perfect bar magnets. The size and shape of these nanometer-scale magnets (nanomagnets) determine their strength, polarization direction, and the ways in which this polarization can be changed by external magnetic fields. The applications of such nanoscale magnets range from mass data storage to cancer treatment and medical imaging.