A variety of technological challenges such as the understanding of water in salt electrolytes (WISE), the development of advanced battery and energy storage technologies, the recycling of desalination brines and a save operation of deep sea boreholes require a thorough microscopic understanding of concentrated electrolyte solutions. However, existing heuristic descriptions of their average excess thermodynamic properties have been derived by extrapolating from the diluted regime, leaving a gap of knowledge for concentrated solutions. From a physico-chemical perspective, the osmotic and average activity coefficients of electrolytes ϕ and ln γ ± , respectively, determine the excess thermodynamic functions of the corresponding aqueous and non-aqueous solutions. Debye and Hückel were the first to describe electrolyte and water properties in dilute electrolyte solutions as a function of ion concentration and electrolyte composition. The theory has later been extended to higher concentrations by Bjerrum, Glueckauf and McMillan and Meyer (see review by Vaslov ). Friedman, Pitzer and coworkers extended the description to more complex electrolyte mixtures such as seawater. A special issue of the Journal of Fluid Phase Equilibria celebrates the 100th anniversary of the findings by Debye and Hückel and provides a summary of recent developments on the topic. In that issue Simonin and Bernard compare several simple activity models including Debye-Hückel, the mean spherical approximation and the Pitzer approach. Earlier, Khan et al. compared four physical descriptions of ln γ ± . While the Pitzer and Bromley approaches describe the activity coefficients of the 1:1 electrolytes well, the methods fail for several important 1:2 electrolytes such as CaCl 2 and MgCl 2 . A recent review by Held provides a critical comparison of the different excess Gibbs energy parametrizations up to high electrolyte concentrations.