Welds made onto in-service pipelines are particularly susceptible to hydrogen cracking. During qualification of welding procedures, limits are often imposed on heat-affected zone hardness (e.g., 350 HV max.) as a way to avoid cracking. The hardness level below which hydrogen cracking does not occur is not a fixed value, but varies as a function of several parameters. The results of previous work resulted in the development of hardness evaluation criteria that can be used to quantify the trade-offs that can be made between HAZ hardness, hydrogen level, and the chemical composition of the materials being welded for welds made onto in-service pipelines. This paper describes a current project that is being sponsored in part by the US Department of Transportation – Pipeline and Hazardous Materials Safety Administration, which is focusing on the further development and validation of these criteria, particularly for modern microalloyed materials and material over a wide range of wall thicknesses. The use of these criteria will reduce the cost and increase the reliability of pipeline modifications and repairs.