It is a commonly encountered statement that shot-noise is the most fundamental and ultimately limiting noise source in any photonic device, because all other noise sources can in principle be reduced to insignificant values. However, this perspective is often of little value, since in practice it is impossible to eliminate other, more trivial, noise sources. It is useful to classify noise sources according to their dependence on the optical power on the detector. Thermal noise and other noise in the electronic circuitry are independent of the optical power. Shot noise current is proportional to the square root of optical power, whereas source intensity noise and thermal noise in the fiber and sensor generate noise current proportional to optical power. Only the noise sources proportional to optical power are of importance if the optical source power can be increased without limit. Often, however, sufficient source power is not available, and shot noise and/or electronic noise become the dominating noise sources. The paper discusses the various noise sources in the light of the above classification. Extra noise sources introduced by fiber- optic amplifiers are included in the discussion.