The use of acoustic cavity fillers to prevent the propagation of air borne and structure borne noise, water and dust into the interior spaces of vehicle structures has been in practice for many years. Various technologies are available to OEMs to provide sealing that will prevent water and dust penetration, maximize performance of vehicle HVAC systems, and minimize the propagation of noise into a vehicle under operating conditions. The current state of the application of these technologies in the automotive, medium duty and heavy duty truck markets will be reviewed. Comparisons will be drawn between two categories of cavity fillers: pre-formed heat reactive expandable parts; and bulk applied chemically reacting polyurethane foam. Typical OEM material requirements will be discussed for these technologies. The pros and cons of these competing technologies, as well as their acoustic performance applied to a full vehicle will be considered. And finally, their typical applications will be reviewed to understand when, where and how these technologies are being used in the market today.