Extensive investigations on co-combustion were carried out using a 0.5 MW pulverized-coal-fuelled experimental furnace with fuel preparation, accompanied by tests on an electrically heated tube reactor. The following additional fuels were utilized within the framework of various projects: biomass such as wood and straw and sewage sludge. The question in focus in the case of the solid feedstock was at first the necessary fuel preparation. The outcome was that biomass (e.g., straw or wood), compared with coal, allows a clearly coarser milling due to its higher content of volatile matter. While the particles must be <1 mm for wood to completely combust, for straw they may be coarser. Regarding combustion behaviour, the major factor beside the volatile matter content is utilized grain size. The delayed ignition of coal/biomass blends could be manifested through measurements in the flame. In the case of sewage sludge, the fine milling and the high content of volatile matter resulted in an accelerated combustion process. Co-combustion also had an effect on emission behaviour. Owing to their high volatile matter content, sewage sludge, straw and wood are suited for application in air and fuel staging with a view to nitrogen abatement. Besides the emission and combustion behaviour, the factors to be taken into account in co-combustion are the operational behaviour (slagging, fouling, corrosion) and the quality of the by-products.