The rules of UCS-66 are some of the most important and yet most misunderstood provisions of the ASME Code. Because of its importance, minimum design metal temperature (MDMT) is one of just a few critical parameters stamped onto vessel nameplates. The rules of UCS-66 guard against vessel failure by brittle fracture, a low probability high consequence event. Brittle fracture failures exhibit “break before leak” behavior and can be catastrophic in that they entail high risks and occur without warning. Carbon steel pressure vessels are most susceptible to brittle fracture during start up, shut down, hydro test (test water that is too cold) and rapid depressurization (auto-refrigeration). Given the consequences of this type of failure, all relevant factors listed in UCS-66 need to be properly addressed. These include consideration of the coldest temperature expected, metallurgy, material thickness, post weld heat treatment, impact testing, applied stress and weld pass size.