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In 1954, Asakura and Oosawa1 explained that nonadsorbing macromolecules can induce attractive forces between colloidal particles. The effective pair interaction between the colloidal particles mediated by the non-adsorbing species is nowadays often termed the Asakura–Oosawa (AO) potential. This special issue of The Journal of Chemical Physics contains a collection of papers related to the concept introduced by Asakura and Oosawa. The late professor Oosawa contributed to this issue with a historical overview on the origin of the Asakura–Oosawa theory. Kurihara and Vincent discussed how, after a lag time of about 20 years, the theory was rediscovered in the 1970s via experimental studies on colloid–polymer mixtures. In the same period, Vrij independently found that excluded volume interactions in a colloidal dispersion containing an additional component have important consequences for the effective interactions, structure, and phase stability.