The following article is an edited transcript based on the Von Hippel Award presentation given by Howard K. Birnbaum of the University of Illinois at Urbana-Champaign on December 4, 2002, at the Materials Research Society Fall Meeting in Boston. The award citation states, “Through innovative use of a wide range of novel experimental tools, Howard K. Birnbaum has made seminal contributions to our understanding of intrinsic point defects, hydrogen in metals, and grain-boundary segregation, especially as these effects relate to mechanical properties. He has also stimulated, directed, and influenced interdisciplinary research throughout the materials community.” This presentation summarizes the major phenomenological observations associated with hydrogen effects on fracture and the viable mechanisms that can account for these observations. It is shown that there are three fracture mechanisms that are consistent with the experimental observations and that the operative fracture mechanism depends on the type of system and the conditions under which it is exposed to hydrogen in aggressive environments.