1. Method for detecting the ingress of air into a system for liquid, and in particular into an extra-corporeal blood circuit of a blood-treating apparatus, the pressure in the system for liquid being measured and the pressure signal measured being analysed, characterised in that cyclic variations in pressure in the system for liquid are measured and the cyclic pressure signal measured is broken down spectrally into a system of functions, and in that the coefficients of the functions are monitored, it being concluded that there has been an ingress of air into the system for liquid if at least one of the coefficients of the functions exceeds or drops below respective preset limiting values.