During the last years parallel object-relational database management systems emerged as the leading data management technology on the market place. These systems are extensible by user-defined data types and user-defined functionality for the data. This work focuses on the efficient parallel execution of user-defined functionality. The main contributions are techniques to support data parallelism for the execution of a scalar function on a large object, and a new technology to provide extensibility with regard to new set-oriented database operations that can efficiently implement user-defined functionality in parallel object-relational database managmeent systems. Some of these techniques have been implemented in the MIDAS prototype or on top of a commercial object-relational database management system.