Superluminal propagation of electromagnetic radiation and photon tunnelling have been studied by a variety of direct and indirect techniques. Especially the use of femtosecond terahertz pulses, whose electric field can be measured directly, has resulted in the (re)discovery of a number of effects in which light propagates over small distances faster than the speed of light in vacuum. Naturally, this brings up the question whether information can be exchanged superluminally. It has been shown in nearly all cases studied that the principle of causality applies to the underlying physical processes. It has been argued, however, that the principle of causality might have no bearing on the question of superluminal information transfer. It will be shown here that all the confusion stems from a vague definition of the concept of information and from ignoring noise. Once the concept of information (and noise) has been defined properly, it can be shown that if the principle of causality applies then useful superluminal information exchange is strictly prohibited.