Field device with - a unit for performing user-defined diagnostic procedures, -- said unit having an input circuit, via which at least one input variable is supplied to the unit, -- said unit having an evaluation unit, --- which is used to derive at least one characteristic variable (K x) using at least one input variable (E x) selected by the user and at least one evaluation procedure (AV x) selected by the user, -- said unit having a monitoring unit, --- which is configured to monitor the derived characteristic variables (K x) using monitoring criteria (UK x) selected by the user, and -- said unit having an output unit, which is configured in such a way that when a monitoring criterion (UK x) occurs it outputs a diagnostic (D x) which is assigned to said monitoring criterion (UK x), and - an interface, which is configured in such a way that a user uses said interface to select, for each user-defined diagnostic procedure, --- the input variables (E x), --- the evaluation criteria (AV x) to be applied to the input variables (E x), --- the monitoring criteria (UK x), and --- to specify the diagnostic (D x) to be output via the output unit, characterized in that the evaluation procedures (AV x) available are implemented in the field device as function blocks which are used in a modular manner, are permanently implemented in the related function blocks in the field device, are activated by corresponding specifications of the user, and are applied to input variables (E x) selected by the user, and in that each monitoring criterion (UK 1... UK n) is provided as a module in the field device. - a unit for performing user-defined diagnostic procedures, -- said unit having an input circuit, via which at least one input variable is supplied to the unit, -- said unit having an evaluation unit, --- which is used to derive at least one characteristic variable (K x) using at least one input variable (E x) selected by the user and at least one evaluation procedure (AV x) selected by the user, -- said unit having a monitoring unit, --- which is configured to monitor the derived characteristic variables (K x) using monitoring criteria (UK x) selected by the user, and -- said unit having an output unit, which is configured in such a way that when a monitoring criterion (UK x) occurs it outputs a diagnostic (D x) which is assigned to said monitoring criterion (UK x), and -- said unit having an input circuit, via which at least one input variable is supplied to the unit, -- said unit having an evaluation unit, --- which is used to derive at least one characteristic variable (K x) using at least one input variable (E x) selected by the user and at least one evaluation procedure (AV x) selected by the user, -- said unit having a monitoring unit, --- which is configured to monitor the derived characteristic variables (K x) using monitoring criteria (UK x) selected by the user, and -- said unit having an output unit, which is configured in such a way that when a monitoring criterion (UK x) occurs it outputs a diagnostic (D x) which is assigned to said monitoring criterion (UK x), and - an interface, which is configured in such a way that a user uses said interface to select, for each user-defined diagnostic procedure, --- the input variables (E x), --- the evaluation criteria (AV x) to be applied to the input variables (E x), --- the monitoring criteria (UK x), and --- to specify the diagnostic (D x) to be output via the output unit, --- the input variables (E x), --- the evaluation criteria (AV x) to be applied to the input variables (E x), --- the monitoring criteria (UK x), and --- to specify the diagnostic (D x) to be output via the output unit,