Summary form only given. When a number of optical signals that fall within the same passband are received, besides the linear crosstalk, interferometric noise (IN) is also produced. The interfering signals can result from a number of sources, such as weak, delayed replicas of the transmitted signal due to reflections in imperfect components, optical signals emanating from different sources, which fall into the passband of the receiver due to four-wave mixing (FWM), switching cross talk or imperfect isolation in add-drop multiplexers. Here we examine a number of methods of evaluation. The first uses the widely quoted Gaussian approximation (GA), in which the joint probability distribution of the interferers is taken to be approximately Gaussian.