It has become increasingly obvious that we should not regard stellar atmospheres as homogeneous and static but must study special effects of forces other than gravity if we are to gain any satisfactorily complete comprehension of the general behavior of these atmospheres. In many stars it may be necessary to investigate certain differential motions in considerable detail before we can make precise determinations of temperature or even reliable quantitative chemical analyses. May I present a brief descriptive outline of the diverse types of atmospheric motions, and then proceed to a short discussion of certain problems of discrete velocities that seem to me especially interesting ? Certain motions of a stellar atmosphere as a whole may more properly be described as stellar motions. They are (1) rotation, which widens the spectrum lines unless the star is seen pole-on ; (2) orbital motions ; and (3) linear motions, which stretch or compress the spectrum according to the Doppler law but do not otherwise alter it. Of the three, only orbital motions in double stars may be expected to change appreciably during a human lifetime. Study of these motions by means of spectral lines formed in stellar atmospheres has played a large part in astronomical spectroscopy but somewhat more attention is now being paid to differential motions within stellar atmospheres. Among the many possible motions within stellar atmospheres, we may first of all distinguish between those connected with radiation in the atmosphere and those of material particles (Table I) . Motions of photons. In stellar atmospheres, the photons that correspond to continuous radiation from the photosphere move predominantly outward, quite in contrast to their short, discontinuous, zigzag paths in the dense, less transparent interior. The