At the end of the last century, quality and especially reusability problems of object-oriented software cropped out. As a remedy, nowadays component based software development resounds throughout the developer communities. Nevertheless the special aspects of testing component based systems often remain overlooked. After characterizing the main differences between object-oriented and componentbased software, this paper firstly surveys some basic concepts of component development and software testing. Then applicable techniques for specification and black-box testing of components are depicted, and particularly contract-based test case specification for component interfaces is emphasized. On this groundwork, some new testing levels and testing roles which have to be played in component based software development are proposed. The paper ends with some prospects on appropriate testing tools.