The recent rise in use of biomass fuels for heat production consequently produces a higher demand of wood at the fuel market, where due to limited availability of economically priced wood, the market for alternative biofuels is growing rapidly, and woody materials of lower quality as well as agricultural raw materials and residues are considered in this regard. The resulting increase in variability of fuel properties poses a major challenge to boiler manufacturers - in particular in the field of small-scale applications, and amongst others ash melting and slag formation in the grate section are a major issue in this regard. In the present thesis, ash melting and slag formation of various types of biomass are investigated, focusing on slag formation in the grate section of small-scale combustion appliances. Raw material and ash analyses are conducted for a comprehensive set of woody and non-woody biomass fuels, and particular importance is attached to the determination of slagging behaviour in practical combustion tests. Findings from real application are compared to results from analyses and lab test methods. In this regard, fuel- and technology related measures to reduce slag formation are investigated, and possibilities for an adequate assessment or classification of the wide range of biomass fuels with respect to their slag formation tendency are analysed.