This guideline brings together manufacturers' and users' expectations with regard to making use of the potential for diagnosis of field devices. Field devices in the meaning of this guideline are sensors and actuators only. Measurement and actuating signals are the most important information from field devices. Further information about the status of the devices is useful and can help the plant operator, DCS system or maintenance staff to take action as required. This guideline summarizes the essential terminology, gives the user some pointers to the use of diagnosis functionality in field devices, describes status signals and what they mean, and lists malfunctions which can be used to set out development goals for device manufacturers with regard to diagnosis functionality of field devices. As the NAMUR recommendation NE 91 already describes the requirements for data exchange with asset management systems in detail, this guideline will not focus on them. Nor does it deal in depth with the software level, where internal device diagnosis results are evaluated. This Part 1 formulates some fundamental aspects of self-monitoring and diagnosis in field devices.