THELI is a powerful tool to reduce optical, near- and mid-infrared astronomical imaging data, supporting more than 100 different single- and multi-chip instruments at professional observatories world-wide. The graphical user interface and internal data model allow the user to concentrate on the highly automated data processing, without being side-tracked by the specific complexities and characteristics of optical and infrared instruments. THELI is fully scalable on multi-processor machines with shared memory, allowing fast processing also of large data sets. The integrated data viewer allows access to any products on drive as well as in memory. User-defined instruments can be added, making THELI interesting for amateur astronomers as well.