This is a critical review of the research on the collector mechanism carried out during the last ten years. The work has been reviewed in three parts. The first part is concerned with the studies on the collector mechanism in sulfide minerals. The discussion centers on the collector action of xanthates and dixanthogens. The nature of the adsorbed film is discussed in the light of recent studies by infrared spectroscopy, electron diffraction, adsorption and calorimetric measurements. The influence of oxygen on the adsorption of xanthate on sulfide minerals is discussed and a critical evaluation made of the evidence for and against the theory that the presence of oxygen is crucial to the formation of a hydrophobic film at the sulfide mineral surface conditioned with xanthate. Recent studies on the stability of the adsorbed collector film are reviewed and correlated with the theories of the collector action of xanthates. The influence of modulating agents, activators, and depressants on the collector action of xanthates on pyrite, pyrrhotite, and marmatite are discussed. Finally, the collector action of xanthates on oxidized lead and zinc minerals has been reviewed and the known basic differences between the nature of the adsorbed species on these minerals and that on sulfide minerals is discussed. The collector mechanism in nonsulfide minerals is reviewed for the following groups of minerals: oxides, silicates, and alumino-silicates, polar salt-type minerals containing alkaline earth metal cations and soluble salts. The collector mechanism of anionic as well as cationic reagents is reviewed in conjunction with the action of modifying agents. The possibilities of employing chelating agents as mineral collectors for nonsulfide minerals are examined. The recent electrokinetic studies on nonsulfide systems in relation to the electrical double layer and nature of the adsorbed species is discussed in detail. In the third part certain new aspects of the collector mechanism, namely the role of impurities and nonstoichiometry of the mineral samples, are discussed. The possibility of controlling the floatabilities of minerals by incorporating impurity centers is examined. Finally, the kinetic factors affecting the collector mechanism are discussed.