A general account, aimed at the practising limnologist, is given of the attenuation of light in natural waters. The most relevant characteristics of the incident light field-its intensity, spectral composition and angular distribution are outlined. The inherent optical properties of the water absorption coefficient, scattering coefficient and volume scattering function are defined, and the contributions of different components of the aquatic system to these are discussed. Taking the properties of the light field and the properties of the water together, the diminution of intensity and the change in spectral quality of the light with increasing depth, in different types of water, are considered. Finally, the characteristic pattern of attenuation of total photosynthetically active radiation, as measured with a quanta meter, in inland and coastal waters is discussed.