The angular correlations of gamma-ray pairs arising from positron annihilation in various solids have been observed. With good resolution, measurements of these angular distributions have been made for a wide variety of metallic elements, several ionic salts, several insulators, and a semiconductor. Some of the results from metals are accountable in terms of the Fermi-gas model of conduction electrons and the assumption of thermalized positrons. Many metals exhibit angular distributions which differ singificantly from the Fermi-gas type, while retaining some of its features. For the noble and transition metals, however, the simple treatment is completely inadequate. Some possible theoretical interpretations of these distributions will be indicated. Measurements made on a group of alkali halides indicate the dominance of the halogen in determining the angular distribution, the width of which increases regularly with the chemical activity. The possibility of using positrons to study Pd - H alloys has been investigated. A measurement of positron mobility in diamond yielded a null result, within the accuracy of the experiment.