This online resource is based on a preparatory course "Mathematics for Engineering" at the International College of the University of Applied Sciences Kaiserslautern in summer term 2009. Its purpose is to make students familiar with the basic principles of complex numbers. Combining explanatory text, exercises and interactive GeoGebra applets, this resource is suitable for both classroom lectures and distance learning. The text and the exercises are available as html format (Firefox recommended) or as printable pdf-files. Before dealing with this resource efficiently, students should have gained basic knowledge of numerical algebra and vector algebra.