In spite of the eradication of smallpox over 30 years ago; orthopox viruses such as smallpox and monkeypox remain serious public health threats both through the possibility of bioterrorism and the intentional release of smallpox and through natural outbreaks of emerging infectious diseases such as monkeypox. The eradication effort was largely made possible by the availability of an effective vaccine based on the immunologically cross-protective vaccinia virus. Although the concept of vaccination dates back to the late 1800s with Edward Jenner, it is only in the past decade that modern immunologic tools have been applied toward deciphering poxvirus immunity. Smallpox vaccines containing vaccinia virus elicit strong humoral and cellular immune responses that confer cross-protective immunity against variola virus for decades after immunization. Recent studies have focused on: establishing the longevity of poxvirus-specific immunity, defining key immune epitopes targeted by T and B cells, developing subunit-based vaccines, and developing genotypic and phenotypic immune response profiles that predict either vaccine response or adverse events following immunization.