Terrestrial mass extinctions have been attributed to a wide range of causes. Some of them are external to Earth, such as bolide impacts (as widely discussed for the K/T boundary) and radiation events. Among radiation events, there are possible large solar flares, nearby supernovae, gamma-ray bursts (GRBs), and others. These have variable intensity, duration, and probability of occurrence, although some generalizations are possible in understanding their effects (Ejzak et al. 2007). Here we focus on gamma-ray bursts (Thorsett 1995; Scalo and Wheeler 2002), a proposed causal agent for the end-Ordovician extinction. These are the most remote and infrequent of events, but by virtue of their power, a threat approximately competitive with, for example, that of nearby supernovae. A GRB of the most powerful type (Woosley and Bloom 2006) is thought to result from a supernova at the end of stellar evolution for very massive stars with high rotational speed. Much of their energy is channeled into beams, or jets, which include very high energy electromagnetic energy, i.e., gamma-rays and X-rays. It is a testament to the power of these events, far across the observable universe, that they were first detected in the 1969–1970 results from monitoring satellites designed to detect nuclear explosions on Earth's surface. It was not until the 1990s, when the distance to the events became known, that their power became apparent. Several such events occur every day in the observable universe. Other kinds of events are also potentially damaging, such as so-called short bursts and solar flares, but rate information is only now beginning to clarify how much threat is likely from such sources.