The evolutionary paths of the massive O and B type stars are often defined by angular momentum transformations that involve circumstellar gas disks. This circumstellar gas is revealed in several kinds of observations, and here I describe a series of investigations of the hydrogen line emission from such disk using detailed studies of five massive binaries and a survey of 128 Be stars. By examining three sets of spectra of the active mass-transfer binary system RY Scuti, I determined masses of 7.1 +/- 1.2 M_sun for the bright supergiant and 30.0 +/- 2.1 M_sun for the massive companion that is hidden by an accretion torus. I also present a cartoon model of the complex mass flows in the system. Using optical spectroscopy and X-ray flux data, I investigated the mass transfer processes in four massive X-ray binaries (a massive B star with mass flowing onto a compact, neutron star companion). The B-supergiant system LS I +65 010 transfers mass via stellar winds. I find the X-ray flux modulates with the orbital period. In the other three X-ray binary systems (LS I +61 303, HDE 245770, and X Persei), an outflowing circumstellar disk is responsible for the mass transfer, and in all three systems, the disk appears to be truncated by gravitational interactions with the compact companion. The disk in the microquasar system LS I +61 303 is limited in radius by the periastron separation and an increase in both H-alpha equivalent width and X-ray flux following periastron may be due to a density wave in the disk induced by tidal forces. Observations of HDE 245770 document what appears to be the regeneration of a circumstellar disk. The disk of X Persei appears to have grown to near record proportions and the X-ray flux has dramatically increased. Tidal interaction may generate a spiral density wave in the disk and cause an increase in H-alpha equivalent width and mass transfer to the compact companion. During the course of the analysis of the X-ray binaries, I developed numerical models for estimating the size of the Be star disks using just the H-alpha equivalent width. Finally, I present the results of a three year spectroscopic survey of both the H-alpha and H-gamma regions of 128 Be stars. I find that the median fractional variation in the equivalent width of the disk emission lines is 15% over a two year period. I also find that two-thirds of the sample displays evidence of Fe II emission or absorption resulting from surrounding circumstellar material. Many candidates for non-radial pulsation and binary systems are also found. Spectra and notes for all of the sample stars are presented in an appendix.