Method for monitoring at least one panel, called a monitored opening panel, which is mounted relative to a frame in such a manner that it can be displaced relative to the frame, characterised in that: - at least one transmitter is fixed in a protruding manner to a front face of a first element chosen from the frame and a monitored opening panel, each transmitter being chosen and arranged to transmit radiation, chosen from electromagnetic radiation and acoustic radiation, in the direction towards a second element: • separate from the first element, • chosen from among the frame and a monitored opening panel, so that the transmitted radiation is at least partially reflected by said second element, - at least one receiver is chosen and arranged to: • pick up radiation, called reflected radiation, coming from at least one transmitter fixed to a first element and reflected by a second element, • and provide signals, called reflection signals, which are representative of at least one characteristic of said reflected radiation, - each transmitter, each receiver, each first element, each second element being chosen and arranged so that at least part of said reflection signals is modified when at least one monitored opening panel is displaced at least from at least one predetermined reference position of that monitored opening panel, - the reflection signals, called instantaneous reflection signals, provided by each receiver are continuously compared to at least one value, called a reference value, - at least one signal, called a displacement signal, which is representative of a displacement of at least one monitored opening panel relative to a reference position is prepared as a function of the result of the comparison between the instantaneous reflection signals and each reference value, said at least one receiver being fixed in a protruding manner to a front face of a third element chosen from the frame and an opening panel, and in such a manner that the radiation transmitted by the transmitter has an axis of transmission which forms an angle α greater than 5° and less than 90° relative to the front face of said first element, and in such a manner that: - the axis of transmission of the transmitted radiation is oriented towards said second element, - the transmitted radiation is reflected by said second element towards the receiver. - at least one transmitter is fixed in a protruding manner to a front face of a first element chosen from the frame and a monitored opening panel, each transmitter being chosen and arranged to transmit radiation, chosen from electromagnetic radiation and acoustic radiation, in the direction towards a second element: • separate from the first element, • chosen from among the frame and a monitored opening panel, so that the transmitted radiation is at least partially reflected by said second element, • separate from the first element, • chosen from among the frame and a monitored opening panel, so that the transmitted radiation is at least partially reflected by said second element, - at least one receiver is chosen and arranged to: • pick up radiation, called reflected radiation, coming from at least one transmitter fixed to a first element and reflected by a second element, • and provide signals, called reflection signals, which are representative of at least one characteristic of said reflected radiation, • pick up radiation, called reflected radiation, coming from at least one transmitter fixed to a first element and reflected by a second element, • and provide signals, called reflection signals, which are representative of at least one characteristic of said reflected radiation, - each transmitter, each receiver, each first element, each second element being chosen and arranged so that at least part of said reflection signals is modified when at least one monitored opening panel is displaced at least from at least one predetermined reference position of that monitored opening panel, - the reflection signals, called instantaneous reflection signals, provided by each receiver are continuously compared to at least one value, called a reference value, - at least one signal, called a displacement signal, which is representative of a displacement of at least one monitored opening panel relative to a reference position is prepared as a function of the result of the comparison between the instantaneous reflection signals and each reference value, - the axis of transmission of the transmitted radiation is oriented towards said second element, - the transmitted radiation is reflected by said second element towards the receiver.