Two different concepts of the Langevin radiation pressure which are ultimately due to Langevin and Brillouin are discussed. The measurement of the radiation force on an absorber at oblique incidence can be regarded as anexperimentumcrucisin this respect. Herrey’s experimental result, which is confirmed by the author’s observations, is in favor of Brillouin. Some fundamentals and limitations of our present knowledge of the Langevin radiation force in nonabsorbing fluids are considered.