Ex vivo enumeration of the absolute frequencies of antigen-specific T cells is key for evaluating the immunogenicity of T cell-based vaccines. Currently there are three methods that are widely used to quantify cellular immune responses: Enzyme-Linked Immuno Spot assay (ELISpot), Intracellular Cytokine Staining assay (ICS) and Tetramer assay. These three different assays offer different information. In this chapter, I discuss the two functional assays, ELISpot and ICS. The ELISpot and ICS assays use short term in vitro stimulation to assay the frequency and cytokine expression profiles of responding cells. The ELISpot assay scores spots of captured cytokine produced by individual cells whereas, ICS uses flow cytometry to profile individual cells for surface markers and the production of cytokines.