For the last 30 yr, interest in vaccinia virus immune monitoring has focused on the use of the vaccinia virus as a recombinant vaccine vector and the potential detrimental effect of antivector immunity on subsequent vaccination with a recombinant vaccinia virus. However, interest in this area has intensified after the publication of reports suggesting that smallpox may be a major pathogen selected for bioterrorist activities. Owing to the unacceptably high incidence of complications induced by previous effective smallpox vaccine strains, alternative safer strains (e.g., modified vaccinia Ankara [MVA]) are being assessed for their antigenicity in clinical trials. The exact immune effector mechanism responsible for vaccine-induced protection to smallpox infection has not been fully elucidated, although it is believed that neutralizing antibody plays a major role. This chapter describes a simple enzyme-linked immunosorbent assay (ELISA) to quantify vaccinia virus antibody titer. Additionally, to define serum-neutralizing activity, both a classical plaque reduction assay and a high-throughput 96-well plate method based on reduction of recombinant vaccinia virus expressed beta-galactosidase is described. Furthermore, details are given for a T-cell proliferation assay, primarily for monitoring T-helper CD4 activity and an enzyme-linked immunospot (ELISPOT) assay for CD8 analysis. The use of reliable immunological assays is vital in assessing the potential efficacy of new vaccines to protect against smallpox infection.