Recent cluster tools, such as cluster tools for chemical vapor deposition processes, have complex and frequently changing module configurations and wafer flow patterns, and strict timing constraints on the residency times at process modules. They also should have appropriate contingency procedures for occurrences of abnormal events such as residency time violation, breakdowns of a process module. It is challenging to rapidly develop and test a cluster tool controller and scheduler for such complex cluster tools. We propose a virtual cluster tool model that can be used for verifying a scheduler or a cluster tool controller. We explain the model architecture and modeling strategies. We also discuss the experience of using the virtual cluster tool model for verifying a cluster tool scheduler.