1. A field device, comprising: means for performing user-defined application-specific diagnostic methods, which said means are a component of a signal processing unit in the field device, an independent unit or a number of decentralized subunits, that are arranged at functionally relevant locations in the field device and connected with one another; an input circuit, via which said means is fed at least one input variable; an evaluation unit, which serves for deriving, on the basis of at least one input variable selected by the user and at least one evaluating method selected by the user, at least one characterizing variable, said at least one evaluating method is implemented in the form of a software module in the field device or executed by circuits specially provided for such, which are correspondingly activated by an input variable of the use: a monitoring unit, which monitors derived characterizing variables on the basis of monitoring criteria selected by the user; an output unit, which, upon the meeting of a monitoring criteria outputs a diagnosis associated with such monitoring criteria; and an interface, via which a user, for each user defined application-specific diagnostic method, selects input variables, selects evaluating methods to apply to the input variables, selects monitoring criteria, and specifies the diagnosis for the output via the output unit.