Thermodynamic modeling is used as a tool to predict the chemistry of ash-forming elements in biomass and waste combustion for corrosion- or deposition-related issues. One major limitation is the lack of comprehensive databases that contain the thermodynamic data of ash compounds and phases formed during combustion. The present paper is a review of the state-of-the-art of the thermodynamic models and databases for ashes of biomass and waste combustion, future developments and the coupling of thermodynamic modeling with modeling of physical properties of molten ash. Recent developments have improved the accuracy for predicting the phase equilibria of alkali salt mixtures. The databases are being expanded by taking into account Ca, Pb, and Zn in the salt mixtures. Chromates have also been included to to predict the stabilities of the corrosion products on boiler heat exchanger materials. The thermodynamic data for silicate systems still lack data for critical subsystems for biomass ashes. The K2O-CaO-SiO2 system which is important for slagging and agglomeration in biomass combustion still needs experimental investigations to make accurate modeling possible. The thermodynamic data of phosphates are discussed and a new modeling approach for molten phosphates is shown for the K2O-P2O5 system. Models for predicting physical properties coupled with the thermodynamic functions of molten silicates and salts are reviewed. Examples of advanced thermodynamic modeling to study ash-related issues in biomass combustion are also shows. (C) 2011 Elsevier B.V. All rights reserved.