The rapidly growing area of information technology constantly demands higher storage densities. Creating ever smaller magnetic structures is an important issue and the magic Terabits per square inch density an ambitious goal. This has led to a dramatically enhanced interest in magnetic nanoparticles and their behavior with a view to being able to design and control their properties to make them suitable for information storage. The toolbox for preparing such particles includes both physical and chemical related approaches with top down and bottom up preparation methods. Preparation, however, has to be accompanied by strict quality control including the arrangements of particles, their shape, structure and magnetic properties. The close interrelation between preparation, properties and control of these particles is emphasized in this Thematic Series. Other applications include the much broader field of sensor technology, especially for medical diagnostics.