Many thousands of tropical species have been settling in the Mediterranean during the last decades. This is the result of congruence between the present Climate Optimum, which is expressed in the warming of the sea and the opening of the contact with the Indo-pacific realm through the Suez Canal and a renewed entry through the Straits of Gibraltar. A historical review shows that tropical biota survived in the Mediterranean till the end of the Pliocene Climate Optimum and that presently we are witnessing a re-colonization of the Mediterranean by Tethyan descendants, rather than an invasion by harmful alien species as happens elsewhere. The limits of this resettling as witnessed today are discussed.