This chapter introduces the dynamic model for DERs in VPPs. Most current methods for VPP participation in power stability support services rely on emergency responses, where DERs quickly adjust their output upon receiving activation commands. Given that VPPs often encompass thousands of DERs with varying characteristics and response latencies, this chapter proposes using equivalent aggregation models to enable VPPs to participate effectively in contingency reserve services. An equivalent aggregation model is first developed to characterize the VPP’s aggregated frequency response, accurately and concisely representing the ability of the DER aggregation to arrest system frequency decline. Based on this model, a performance-to-cost map is constructed, detailing the minimum cost required to achieve equivalent performance for various aggregation models. Numerical case studies validate the effectiveness of the proposed equivalent aggregation model.