In the discussion which follows, there are presented three areas of the Naval Research Laboratory's study of absorption effects in the spectra of discrete sources produced by interstellar hydrogen. The first topic presented is the theory of the two mechanisms which are operative in producing the observed absorption effects. The second area of the discussion presents the observations and their interpretations for sources which lie in directions where the effects of galactic rotation are an important aid in the interpretation of the observed absorption. When galactic rotation effects are small, special techniques must be developed to supplement the absorption effect in order to determine distances to the discrete source. An example of the latter technique is contained in the third topic presented here, in a study of the source Sagittarius A. All the data discussed here were obtained with the Naval Research Laboratory's 21-cm. radiometer in conjunction with the 50-ft. antenna. A signal band width of 5 kc. (1 km./sec.) was employed throughout.