The behavior of materials at cryogenic temperatures has become an important area of our technology during the last decade. In recent years, a wealth of data, particularly mechanical property data, has been generated on the properties of numerous materials at very low temperatures. These data have been primarily of a phenomenological nature; significantly less attention has been devoted to gaining an understanding of these phenomena. As our first entry into the area of cryogenics, the phenomenological approach was justified, particularly since many materials were suitable for cryogenic service and merely required testing to determine design properties. However, future developments in the cryogenic materials field will require more thorough fundamental understanding of behavior. The objective of this symposium was to provide the technical community with a convenient base for developing such an understanding. In the first paper, Kula and DeSisto review the factors governing plastic behavior at low temperatures. Following a phenomenological presentation of behavior, such as serrated yielding and effect of crystal structure, the authors present fundamental discussions of the mechanisms associated with these characteristics. Wessel presents a discussion of the basic factors governing fracture of metals and then proceeds to show that fracture data can be used to establish performance characteristics of structures.