Magnetic colloids are very promising stimuli-sensitive materials for drug delivery because of their magnetic field responsiveness. An applied magnetic field can concentrate the magnetic nanodevice at the desired site, keeping it there until the drug is totally released. Furthermore, the versatility of magnetic colloids allows the design of multifunctional nanoplatforms with combined therapy and imaging functionalities against diseases with high morbidity. This book summarises the most important advances in the formulation and functionalisation of such magnetic micro- and nano-particles, and also describes the recent advances on their use in biomedicine.