Method for monitoring a brake system for motor vehicles, which can be controlled in a so-called "brake-by-wire" operating mode both by a vehicle driver and also independently of the vehicle driver, with • a hydraulic actuation device operated by means of a brake pedal with a brake master cylinder, to which wheel brakes are connected, especially in at least two brake circuits (12, 13), and a simulation device, which comprises at least one elastic element and provides the vehicle driver with a pleasant pedal feel, wherein a third measurement variable (P Pedal) characterizing the pressure of the brake master cylinder and a fourth measurement variable (S Pedal) characterizing an actuating travel of the brake pedal are detected, • an electrically controllable pressure generating device, which is formed by a cylinder-piston assembly, whose piston can be operated by an electromechanical actuation device, and which is connected or can be connected to the wheel brakes, wherein a first measurement variable (P DBE) characterizing the pressure of the pressure generating device and a second measurement variable (S DBE) characterizing a position or location of the piston are detected, and • an electronic control and regulating unit, wherein for monitoring a functional state of the brake system the detected measurement values of the first and the second measurement variable (S DBE, P DBE) are compared with a specified characteristic field of first and second measurement variables, characterized in that an actual system stiffness of the pressure generating device is determined from the measurement values of the first and second measurement variables (P DBE, S DBE) as the derivative of the first measurement variable (P DBE) with respect to the second measurement variable (S DBE) or vice-versa, in that for monitoring the functional state of the brake system the detected measurement values of the third and the fourth measurement variables (P Pedal, S Pedal) are compared with a specified characteristic field of third and fourth measurement variables, in that an actual system stiffness of the actuation device is determined from the measurement values of the third and fourth measurement variables (P Pedal, S Pedal) as the derivative of the third measurement variable (P Pedal) with respect to the fourth measurement variable (S Pedal) or vice-versa, and in that an overall stiffness (C_GES), which is monitored, is determined from the actual system stiffnesses (C1, C2) of the actuation device and the pressure generating device. • a hydraulic actuation device operated by means of a brake pedal with a brake master cylinder, to which wheel brakes are connected, especially in at least two brake circuits (12, 13), and a simulation device, which comprises at least one elastic element and provides the vehicle driver with a pleasant pedal feel, wherein a third measurement variable (P Pedal) characterizing the pressure of the brake master cylinder and a fourth measurement variable (S Pedal) characterizing an actuating travel of the brake pedal are detected, • an electrically controllable pressure generating device, which is formed by a cylinder-piston assembly, whose piston can be operated by an electromechanical actuation device, and which is connected or can be connected to the wheel brakes, wherein a first measurement variable (P DBE) characterizing the pressure of the pressure generating device and a second measurement variable (S DBE) characterizing a position or location of the piston are detected, and • an electronic control and regulating unit, wherein for monitoring a functional state of the brake system the detected measurement values of the first and the second measurement variable (S DBE, P DBE) are compared with a specified characteristic field of first and second measurement variables, characterized in that an actual system stiffness of the pressure generating device is determined from the measurement values of the first and second measurement variables (P DBE, S DBE) as the derivative of the first measurement variable (P DBE) with respect to the second measurement variable (S DBE) or vice-versa, in that for monitoring the functional state of the brake system the detected measurement values of the third and the fourth measurement variables (P Pedal, S Pedal) are compared with a specified characteristic field of third and fourth measurement variables, in that an actual system stiffness of the actuation device is determined from the measurement values of the third and fourth measurement variables (P Pedal, S Pedal) as the derivative of the third measurement variable (P Pedal) with respect to the fourth measurement variable (S Pedal) or vice-versa, and in that an overall stiffness (C_GES), which is monitored, is determined from the actual system stiffnesses (C1, C2) of the actuation device and the pressure generating device.