Vapor intrusion (VI) is emerging as a significant concern at contaminated sites around the country. Soil and groundwater contamination is able to migrate in a vapor form into occupied structures, and even at relatively low concentrations, these vapors are harmful to human health. Investigating the VI pathway typically involves knowing the extent of soil and groundwater contamination, understanding building construction, and sampling soil gas below or adjacent to a building. If necessary, indoor air samples can also be collected. Vapor intrusion investigations are required frequently at leaking underground storage tank (LUST) and chlorinated solvent release sites, and are frequently recommended as part of the environmental transactions process (Phase II and BEAs) to limit liability and determine pre-existing contamination. Additionally, property owners nearby present/former gas stations or dry cleaning operations may want to presumptively investigate vapor intrusion, as VI investigations were typically not required in past regulations. If a VI assessment indicates that VI is occurring and has the potential to harm human health, Vapor Mitigation or remediation of the vapors will be necessary.