This chapter deals with the main properties of solvents, solutions and polymer solutions. The definition of solubility is provided, and the most important physico-chemical properties of solvents are introduced. The solute–solvent interactions, depending on their chemical composition, are discussed, focusing on the parameters that control the solubility of a solid in a liquid. The solubility of gases in liquids and the Henry's law are then discussed. The Hildebrand solubility parameters are presented as a guideline to predict the miscibility between two compounds. Basic definitions of polymers are then provided, focusing on the molecular weight, the polydispersity and the structural parameters. Finally, the solubility of polymers is approached through the Flory–Huggins theory, discussing the χ parameter as a tool for evaluating the goodness of fit of a solvent with a solute.