A main issue for the development of gasification processes for solid fuels to produce gas for electricity production at the small to medium scale is the upgrading of the raw gas to a quality required by engines. A main problem to be solved is to convert the tar formed in the gasifier to useful gas components by an economical, efficient and reliable technique. Older techniques for such systems need separation and periodical cleaning from tar that condenses in ducts between the gasifier and the engine. Other issues that need to be addressed are effects from contaminants both on the performance of the gasifier and upgrading system and on the emissions by the flue gas. Development should preferably aim at technical solutions that will accept a wide range of fuels, with varying contents of chlorine, nitrogen and sulphur. The results from this project were both the development and implementation of a new technique for tar-decomposition and a large amount of basic information about the behaviour of catalytic materials and their applications that can be used for upgrading of the raw gas by different process concepts. A complete biomass-to-electricity unit at a scale of 100 kWth with a new design suitable for the small and intermediate scale was thus erected and operated in a test programme, using a reversal flow tar converter (RFTC) in a new process concept for decomposition of tar. Progress was made and further experience was obtained on how tar contents can be reduced by different catalysts and in different process schemes. Important knowledge and data was revealed concerning the detailed kinetics of tar compounds and about the intrinsic behaviour of dolomite and nickel-based catalysts when used for conversion of tar. New fundamental information on sulphur adsorption on nickel and on effects from chlorine on catalysis by dolomite will be useful in the evaluation of process modifications in catalytic hot gas cleaning. The SPA method for tar analysis and direct mass spectrometry was proven to be a very useful tool in the development and testing of gasification processes. The SPA method is currently being discussed to be adapted as an international standard method for tar analysis and it will be included as a candidate for such acceptance in organised work on standardisation, supported by EC and IEA.