Abstract The capacity of hydrothermal fluids to transport metals in concentrations sufficient to form ore deposits is due in large part to the polar nature of the water molecule and the ability of metals to form strong aqueous complexes with a number of ligands commonly found in nature. In this paper, we review the properties of hydrothermal liquids and vapors, show how the hard/soft acid/base (HSAB) principle can be used to predict why certain metals form strong complexes with particular ligands, and review the experimental data on the aqueous speciation of a selection of base, precious, and critical metals in high- and low-density hydrothermal fluids. Based on these data, we identify the important complexes for each metal and determine the physicochemical conditions under which they may predominate and thereby control hydrothermal metal transport. This information is used to quantitatively determine the solubility of the main ore minerals in hydrothermal liquids and vapors, and evaluate the mechanisms of metallic mineral deposition (cooling, fluid mixing, boiling, and fluid-rock interaction) in selected ore-forming systems.