Validating instruments is a complex procedure and it needs time and involvement from psychologists’ part. From the point of view of executing the operations specific to the workplace, tasks and activities in order to obtain the wished / desired performances, the human operator will have to be rigorously psychologically evaluated. The evaluation is based on verifying those aptitudes, habituations and capacities necessary for a good development of the workplace activities. Hence, the paper highlight highlights the role validating procedures play using the criteria (tasks, activities, operations and actions) and the methodology of validation.