The application of very-long baseline interferometry (VLBI) techniques at meter and decameter wavelengths has proven to be a unique tool for studying a number of different phenomena. Because of the high angular resolution which we achieve with VLBI, candidate objects for study include the compact nuclei and "jets" associated with quasars and radio galaxies, pulsars, small knots in galactic supernova remnants, and nonthermal planetary sources. Our investigations have been aimed at determining the emission spectra of these objects, measuring their apparent angular size, and mapping their structure. We expect that these properties will be interrelated, particularly at or below the frequencies at which their spectra peak. As various components of a radio source become self-absorbed the apparent angular structure must change quite radically. Concomitant investigations of transmission properties of the various media which intervene between the source and the observer must be carried out. Often these investigations of the interstellar, interplanetary, and ionospheric media are as interesting as the study of the radio sources themselves.