According to the invention, a monitoring device (12) is created for monitoring a thinning of at least one semiconductor wafer (4) in a wet etching unit (5), wherein the monitoring device (12) comprises a light source (14), which is designed to emit coherent light of a light wave band for which the semiconductor wafer (4) is optically transparent. The monitoring device (12) further comprises a measuring head (13), which is arranged contact-free with respect to a surface of the semiconductor wafer (4) to be etched, wherein the measuring head (13) is designed to irradiate the semiconductor wafer (4) with the coherent light of the light wave band and to receive radiation (16) reflected by the semiconductor wafer (4). Moreover, the monitoring device (12) comprises a spectrometer (17) and a beam splitter, via which the coherent light of the light wave band is directed to the measuring head (13) and the reflected radiation is directed to the spectrometer (17). The monitoring device (12) further comprises an evaluation unit (18), wherein the evaluation unit (18) is designed to determine a thickness d(t) of the semiconductor wafer (4) from the radiation (16) reflected by the semiconductor wafer (4) during thinning of the semiconductor wafer (4) by means of a method that is selected from the group consisting of a 1D-se FDOCT method, a 1D-te FDOCT method and a 1D-se TDOCT method.