The impact test exemption rules of UCS-66 are some of the most important and yet most misunderstood provisions of the ASME Code. Because of its importance in determining safe vessel operation, 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. Although infrequent, brittle fracture failures are catastrophic in that they entail high risks and occur without warning. Carbon steel pressure vessels are generally most susceptible to brittle fracture during start up, shut down and hydro test. Given the consequences of this type of failure, it is important that all the relevant factors listed in UCS-66 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.