Abstract Because in the past vaccines were considered inherently safe, only limited nonclinical testing used to be required. However, from about 2003 more stringent nonclinical testing guidelines were introduced for novel vaccines (and, in some cases, marketed vaccines attempting registration in new markets) given that vaccines are typically administered to healthy individuals, often infants. These safety considerations become even more important as technologies allowing the development of novel vaccine types are developed, such as DNA vaccines and those delivered in vector systems such as adenoviral vectors. The use of adjuvants other than the standard aluminum-based ones and novel delivery systems such as electrostimulation have also led to additional testing requirements. This chapter describes the current regulatory expectations for the preclinical development of vaccines.