Information interaction of Cosmic Civilizations means both reception and sending of interstellar messages (IMs). Search of IMs has sense only in the case when those who search assume existence of IM sources. Similarly, sending IMs is meaningful only if there is a hope for existence of those who can detect these IMs. Thus, the concept “Interstellar Messages” applicable to both SETI (treated as searches of IMs) and METI (treated as sending of IMs). The given concept can be considered as a specific invariant of transformation SETI ? METI. As such an approach there is no division into Terrestrial and Extra-Terrestrials and opposition of the Earth and Cosmos that allows considering attempts of information interaction of intelligence space systems as something universal, inherent to the inhabited Universe. Besides, it is important to note that at such an approach one does not talk any longer about “Messages from the Earth” but about Interstellar Messages.