This chapter describes how behavioral models function within OTS. It does not describe the models themselves. For this the reader is referred to the respective references. The structure and modularity of behavioral models is strongly correlated with the hierarchical approach of Michon discussed in section 1.1, and to the model factories as discussed in section GTU characteristics generator. This chapter discusses three model layers in descending hierarchy: strategical planner, tactical planner and the car-following model. Only the strategical planner and tactical planner are mandatory, while a model structure may also contain more sub-components, depending on the used components. The final two sections of this chapter discus the default tactical planner which is the Lane change Model with Relaxation and Synchronization (LMRS) and how parameters are used in OTS. As mentioned, not the model itself, but the structure and modularity of the tactical planner are discussed.