ABSTRACT - Nowadays, most optical communication links rely on lasers. They show narrow line widths, are relatively fast and are well understood. For short-reach, low-cost applications, however, they provide some obstacles, such as eye-safety issues, complex driving circuits etc. Therefore, more and more short-reach applications use Light-Emitting Diodes (LEDs) as light sources despite their inferior characteristics compared to lasers. This talk gives an overview on types of LEDs, applications and requirements for LEDs in short-reach communication systems and explains some interesting nonlinear modulation properties we have obtained at our research throughout the years. The LEDs seems to be a simple device, but there is not everything fully understood...