System level modeling is becoming a necessity in all areas of engineering design. As systems grow in complexity, designers may increasingly rely on commercial off-the-shelf (COTS) components. Frequently, these components are described at a high level of abstraction (behaviorally) that complicates fault testing.This paper discusses the trade-offs of using behavioral components in a design, specifically as it relates to fault simulation. We investigate important issues such as timing, and examine the need to internally fault behavioral models. We then present our multi-level concurrent fault simulator (MCS) that can accept any combination of gate level and behavioral models using a single kernel. Our kernel propagates faults through behavioral components deterministically. Finally, we present simulation results of multi-level models to demonstrate our simulator's capabilities and performance.