Method for monitoring the hydraulic supply system of an at least partially hydraulically operating plastic processing machine, in particular an injection molding machine, wherein the machine has at least one hydraulically driven axis (2, 3, 4, 5) with at least one piston-cylinder element, wherein the hydraulic supply system comprises at least one driven hydraulic pump with which hydraulic oil is delivered and fed into the at least one piston-cylinder element in order to carry out a movement of the axis (2, 3, 4, 5), characterized in that, in order to monitor the aging process or an internal leakage of the hydraulic pump or a hydraulic component of the hydraulic system, it comprises the following steps: a) Prior operating the plastic processing machine: a1) determination of the nominal flow rate (Q 0) of the hydraulic pump as a function of at least one operating parameter (n, α) of the hydraulic pump and a2) determination of the nominal displacement speed (v 0) of the hydraulically driven axis (2, 3, 4, 5) when the piston-cylinder element is operated with a predetermined nominal flow rate (Qa); b) During the operation of the plastic processing machine: b1) When the hydraulically driven axis (2, 3, 4, 5) is operated: Measurement of the at least one operating parameter (n, α) of the hydraulic pump as well as the actual displacement speed (v Ist) of the hydraulic axis (2, 3, 4, 5); b2) Comparison of the actual displacement speed (v Ist) or a variable directly related thereto with the nominal displacement speed (v 0) or a variable directly related thereto, which result with the at least one operating parameter (n, α) of the hydraulic pump measured according to step b1), and storage and/or output and/or display of the result of the comparison. wherein an indication is output if the comparison according to step b2) shows that a deviation between the actual flow rate (Q Ist) and the nominal flow rate (Q 0) or between the actual displacement velocity (v Ist) and the nominal displacement velocity (v 0) has been determined which exceeds a predetermined tolerance. a) Prior operating the plastic processing machine: a1) determination of the nominal flow rate (Q 0) of the hydraulic pump as a function of at least one operating parameter (n, α) of the hydraulic pump and a2) determination of the nominal displacement speed (v 0) of the hydraulically driven axis (2, 3, 4, 5) when the piston-cylinder element is operated with a predetermined nominal flow rate (Qa); a1) determination of the nominal flow rate (Q 0) of the hydraulic pump as a function of at least one operating parameter (n, α) of the hydraulic pump and a2) determination of the nominal displacement speed (v 0) of the hydraulically driven axis (2, 3, 4, 5) when the piston-cylinder element is operated with a predetermined nominal flow rate (Qa); b) During the operation of the plastic processing machine: b1) When the hydraulically driven axis (2, 3, 4, 5) is operated: Measurement of the at least one operating parameter (n, α) of the hydraulic pump as well as the actual displacement speed (v Ist) of the hydraulic axis (2, 3, 4, 5); b2) Comparison of the actual displacement speed (v Ist) or a variable directly related thereto with the nominal displacement speed (v 0) or a variable directly related thereto, which result with the at least one operating parameter (n, α) of the hydraulic pump measured according to step b1), and storage and/or output and/or display of the result of the comparison. wherein an indication is output if the comparison according to step b2) shows that a deviation between the actual flow rate (Q Ist) and the nominal flow rate (Q 0) or between the actual displacement velocity (v Ist) and the nominal displacement velocity (v 0) has been determined which exceeds a predetermined tolerance. b1) When the hydraulically driven axis (2, 3, 4, 5) is operated: Measurement of the at least one operating parameter (n, α) of the hydraulic pump as well as the actual displacement speed (v Ist) of the hydraulic axis (2, 3, 4, 5); b2) Comparison of the actual displacement speed (v Ist) or a variable directly related thereto with the nominal displacement speed (v 0) or a variable directly related thereto, which result with the at least one operating parameter (n, α) of the hydraulic pump measured according to step b1), and storage and/or output and/or display of the result of the comparison. wherein an indication is output if the comparison according to step b2) shows that a deviation between the actual flow rate (Q Ist) and the nominal flow rate (Q 0) or between the actual displacement velocity (v Ist) and the nominal displacement velocity (v 0) has been determined which exceeds a predetermined tolerance.