The overall objective of the work described in this paper was to determine the behavior of wood ash under entrained-flow gasification conditions. Experimental work in atmospheric and pressurized entrained-flow gasification simulators, combined with thermodynamic equilibrium calculations, has shown that wood ash is not prone to form a molten slag at typical operating conditions of (pressurized, dry-feed, oxygen-blown) entrained-flow gasifiers, in spite of the presence of a relatively high amount of low-melting alkaline elements. This appears mostly due to the formation of mainly high-temperature-melting compounds (e.g., CaO) and only a small fraction of Ca silicates, which are characterized by a lower melting temperature. Phosphor and silicon may contribute to creating a higher melt amount, whereas low-melting alkali metal compounds are mostly partitioned into the vapor phase. Experiments, as well as modeling work performed for three types of wood, have shown consistent results. Addition of a fluxing agent is a promising option to improve the slagging behavior of wood-based systems by reducing the melting point of the slag. Moreover, thermodynamic calculations have shown that slag recycle may represent a feasible option in order to obtain sufficient slag coverage of the refractory wall despite the low ash content of woody fuels (typically I order of magnitude lower than in coal). In the present work, the determination of slag viscosity, a parameter critical for continuous operation of a slagging gasifier, has been addressed as well. The results of modeling work, showing the inapplicability of predictive formulas developed in the past for coal slags to wood-based slags, underline that further work is required to allow for a quantitative assessment of the slag viscosity as a function of slag composition and temperature.