Observations with milliarsecond resolution using the Navy Optical Interferometer have been obtained for a number of stellar systems which include high-mass binaries, eclipsing binaries, and radio stars. These observations also reveal the previously unseen companions in single-lined spectroscopic binaries via directly measured flux ratios. We will present examples of published and ongoing research efforts of these systems to illustrate how an optical interferometer contrib\-utes to our knowledge of stars, their environment, and companions. These studies include a conclusive revealing of the previously unseen companion in the single-lined binary Φ Herculis, the direct determination of orbital parameters in the wide and close orbits of Algol, and revealing the orbit of β Lyrae with spatially resolved images of the Hα emission.