Janus particles, named after the double-faced Roman god, are compartmentalized colloids with two sides of different chemistry or polarity. This makes them a unique class of materials among micron- or nanosized particles. Due to this special non-centrosymmetric feature in the particle architecture, the synthesis of Janus particles remained challenging for a long time. However, major progress concerning their preparation in useful amounts has been achieved in recent years. The interest in these particles arises from their fascinating hierarchical superstructures in solution and from the fact that demanding problems in materials science, biomedicine and in the field of highly specific sensors can be tackled with this class of particles with advanced properties. This article highlights preparation pathways, self-assembly processes and various pursued applications.