This paper discusses the fundamentals, mechanical engineering and commercialisation of co-current moving bed gasifiers. The process engineering aspect stresses that gasifiers can be designed to obtain tar-free product gas. Based on the fuel properties, the fire hearth has to be adjusted, with an annular throat concept applied in the case of larger capacities and fine fuel. Models are presented for maximum capacity calculations of gasifiers with respect to tar production and for reduction zone design. From a mechanical engineering aspect fuel handling, materials selection, ash removal, gas treatment and utilization as well as safety risks are discussed. The present state of gasifier commercialization is given. The economic evaluation leads to the conclusion that gasification is a viable option for direct heating and co-generation.