This chapter shows how multiclass open QN models can be used to analyze the scalability of multi-tiered e-business services. The workload of these services is characterized at the user level. User models such as the Customer Behavior Model Graph (CBMG) are used to characterize the way customers navigate through the various e-business functions during a typical visit to an e-commerce site. This user-level characterization can be mapped to a request-level characterization used by QN models. The models are used for capacity planning and performance prediction of various what-if scenarios.