In a systematic way, the impact of chaos present in transmitters for chaos based communications is discussed; based on arguments from information theory. Conditions for the optimality of communication systems involving chaos in the transmitter are developed. Based on these results which apply quite generally, different approaches to chaos based communications will be analyzed. In particular, what will be shown is that optimal performance in Shannon's sense implies that no information about the chaos (in the Shannon/Kolmogorov-Sinai sense) reaches the receiver. There are several ways to achieve this, some following previously known arguments, and some others that perhaps open new perspectives. In particular, it is shown that the emission of chaotic signals by the transmitter is not precluded in general, but for a number of common communication channels.