Working group 1 aims at understanding the role of CRMs in high-tech materials applied in extreme aggressive conditions to ground the baseline of the various WGs thus, creating a path for the Action efforts. Presently, there is a wide array of materials that have been developed via the trial and error approach, gathering knowledge from conventional applications. Conversely, a more rational strategy is needed to substitute CRMs in materials fabrication. It is clear that is crucial to understand the particular role and impact of the specific CRMs used either as alloying element or additive and why its presence is related to the overall material performance and durability. There is the need to implement rational and efficient strategies to concisely understand and define CRMs impact on materials design and fabrication and how this impact the material performance in extreme conditions. For example the precise role of niobium in high-strength low-alloy steel is not completely understood but this could be crucial for successful substitution of Nb without compromising the material behaviour. This becomes more critical when materials are requested to operate in extremes of the most harsh environments.