This report discusses the current status of the joining technology of molybdenum-based metals and alloys. An overview is provided of the phenomenon of temperature influenced ductile-to-brittle transition@ which will lead the reader to an appreciation of the often conflicting events associated with maintaining elevated temperature properties while improving lower temperature ductility. This discussion includes the accepted theories of dislocation mobility in body-centered cubic (BCC) metals and the role of strain rate as well as the effects of interstitial and substitutional elements. A summary is provided of current experiments which confirm and illustrate the relevance of these theories. Information of practical significance is included which will assist in both the design and utilization of molybdenum-base metals.