The composition and structure of catalysts for catalytic pyrolysis are usually based on either conventional petroleum reforming or cracking catalysts or research materials derived there from. These systems have distinct advantages in terms of scale, availability of supply and cost since they are manufactured on the scale of millions of tonnes per annum. Conventional petroleum catalysts are roughly divided into two classes, alumina-silicates for cracking (i. e. formation of small chain hydrocarbons from long chain hydrocarbons)160 and transition metal catalysts for reforming (isomerisation and aromatisation).161 Note that because of the complex nature of these reactions, separating the effects of the catalysts into the simple roles indicated here is overly simplified. The alumina-silicate materials can be divided into several different classes as described below. Petroleum cracking catalysts tend to be alumina or silica supported precious metals or nickel based materials and because of the cost and lifetime of the precious metal catalysts in pyrolysis environments, investigation of these precious metal catalysts is limited to mainly academic work. Cost is an important issue in pyrolysis. Environments are harsh and both poisoning and sintering mitigate against achievement of long catalyst lifetimes. As a possible solution to this low cost materials are of importance and many studies are being carried out in commonly found inorganics such as naturally occurring zeolites (a complex aluminosilicate as described below) and clays. Because of their low cost and plentiful supply, carbonate materials are often used as catalysts in pyrolysis although their use has been largely superseded by the use of aluminosilicate materials. The various types of catalyst are described below. All of these would have a different range in functionality and this should be used to match the various pyrolysis materials used. For example for heavy oils a catalyst with combined cracking, isomerisation and aromatisation catalyst may be used. For a biomass pyrolysis a catalyst with strong de-hydroxylation capability might be preferred. For these reasons, a great deal of research effort is placed in developing the catalysts for a particular raw material as well as the pyrolysis reactor used.