Optical signal-to-noise ratio (OSNR) monitoring methods and apparatus are described. A tunable optical filter filters an optical channel containing an optical signal and noise. The total signal and noise power at the output of the filter is measured as the transmittance passband of the filter is varied and the maximum and minimum powers are determined. The ratio between the maximum and minimum powers is then used to determine the OSNR of the optical channel, which, for example, can be a wavelength channel in a wavelength division multiplexing (WDM) system. The ratio of the maximum signal power to the minimum signal power and the ratio of the maximum noise power to the minimum noise power are pre-determined based on the signal modulation format type and filter passband characteristics. Because the OSNR monitoring method and apparatus rely on information obtained after spectrally filtering the signal and noise, their operation is independent of any transmission effect that does not affect the optical power spectra of the signal and the noise or affects them in a known manner. For example, effects such as chromatic dispersion (CD), polarization-mode dispersion (PMD), and changes in the signal degree of polarization (DOP) and noise DOP will not affect the OSNR reading thus obtained.