An Intrusion Detection System (IDS) is a program or set of programs that detects unauthorized uses of computers. As the use of IDSs increases, the need for sound methodologies and tools for testing IDSs is also growing. This paper discusses the authors' development of a software platform that can be used to simulate intrusions in the course of testing IDSs. The platform, which is built on top of the Unix expect and Tcl packages, allows a user to create their own intrusion scripts. The platform supports the parallel execution of several scripts, so that coordinated intrusions with multiple targets can be simulated. Synchronization mechanisms in the platform are provided so that parallel script tests are reproducible. The authors are using the platform to create an initial set of test scripts, and intend to extend this initial script set into a benchmark suite for IDSs. Research in testing methodology for IDSs is guiding the development of the script set. The paper includes results from tests on actual IDSs using the current script set.