The progress in the understanding of the high velocity component of the interstellar medium is reviewed. Emphasis is on the velocities seen outside the plane of the Milky Way, of gas in the galactic halo.
The extensive measurements of interstellar absorption lines in the late 1930’s demonstrated that the “stationary” lines in stellar spectra were extremely narrow and in many cases showed fine structure at the 10 km/s level. The quantisation of these data had to wait until about 1950. Adams (1949) presented data on Ca II lines toward some 300 stars and his compilation, supplemented with Na I data, led Routly and Spitzer (1952) to investigate the systematics of the Ca and Na abundances. Contrary to the non-solar abundance ratio of the majority of the interstellar clouds, the “high velocity” absorptions, defined by Routly and Spitzer as differing in velocity by more than 20 km/s from the local standard of rest, showed a near solar Ca/Na abundance ratio.