This review discusses the legal requirements and property specifications for blowing agents in different applications. It highlights the effects of changing blowing agents including the need for reformulation. Many new polyols, isocyanates and surfactants are being developed to overcome problems. Each type of blowing agent is described. Key environmental and physical properties are listed, together with advantages and limitations. Foams are described by types and by applications. An additional indexed section containing several hundred abstracts from the Rapra Polymer Library database gives useful references for further reading.