Since vaccinia virus was first used to protect against smallpox in the eighteenth century, live attenuated vaccines have proved to be highly effective in reducing the morbidity and mortality caused by many human viral pathogens. Contemporary live viral vaccines are designed using several different strategies to achieve attenuation. These basic principles and approaches are illustrated by vaccines to prevent rotavirus, influenza and varicella-zoster virus infections that are described in this chapter. As shown from the experience with these three vaccines, contemporary live attenuated viral vaccines have had a major impact on disease caused by these ubiquitous human pathogens.