Operational requirements and objectives for HSIs are typically derived from the need for cost-effective power production, nuclear safety, technical requirements imposed by plant processes, the level of automation, and the need to protect workers, the public and the environment. HFE work during the early phases of plant design should focus on how systems will be operated by the users, including interfaces and their interoperability with other systems. These requirements will establish how well and under what conditions the system must perform and will thus form part of the basis for HSI design and technology selection.