One of the most active research areas in functional materials is the chemical synthesis of magnetic nanoparticles. Their potential in biomedical and technological applications has motivated the search for new forms of magnetic nanoparticles with additional chemical and physical functionalities. This review highlights recent advances in the high temperature solution-phase synthesis of monodisperse, multicomponent magnetic nanoparticles. Particular emphasis is put on iron-based metal, oxide, alloy and composite nanoparticles and their biomedical applications.