Extra dimensions have "existed" for many years in string theories. In recent years, two lines of thought have suggested that effects of extra dimensions can be measurable at future experiments. In the Arkani-Hamed-Dvali-Dimopoulos scenario, extra dimensions can be close to the millimeter scale, provided they are only "seen" by gravity. This leads to a near-continuum of massive gravitons, with masses from milli-eV up to a few TeV. In the Randall-Sundrum scenario, our (3+1)-dimensional world is separated from a shadow world by a distance d in a fifth dimension, associated with heavy (widely separated) gravitons at the TeV scale. In both scenarios, massive gravitons can be produced in high-energy collisions, resulting in events with missing energy. They can also contribute as intermediate fields, leading to distinctive signatures. Some such effects will be discussed.