Abstract: This paper introduces and motivates the application of parameterization to behavior trees. As a framework, behavior trees are becoming more commonly used for agent controllers in interactive game environments. We describe a way by which behavior trees can be authored for acting upon functions with arguments, as opposed to being limited to nonparametric tasks. We expand upon this idea to provide a method by which a subtree itself can be encapsulated with an exposed parameter interface through a lookup node, which enables code reuse in a manner already exploited by object oriented programming languages. Parameterization also allows us to recast Smart Events (a mechanism for co-opting agents to perform a desired activity) as behavior trees that can act generically upon groups of typed agents. Finally, we introduce a tool called Topiary, which enables the graphically-oriented authoring of behavior trees with this functionality as part of a broader testbed for agent simulation.