The repetitive-interspersed DNA sequences constitute a large part of the genome of eukaryotic and prokaryotic organisms. The mobility of these sequences was first described by Barbara MacClintock in the fifties who, by studying the inheritance of the colour and distribution of the pigmentation in maize, showed the existence of genetic elements with the ability to transpose along the genome. In the present study we review some of the molecular, philogenetic and functional characteristics of these mobile elements with special emphasis on LINE elements. The analysis of the presence of mobile elements in host organisms indicates that they co-evolve with the host to avoid or mitigate the deleterious effect of their insertion and even to provide potential beneficial features. Presently, these elements are beginning to be considered as playing important roles in the evolution of the organisms within a symbiotic framework.