Horizontal gene transfer as well as mutations, ge- nomic reorganization and gene loss is one of major driving forces of speciation and evolution of bacteria. A notion of definition of "species genome" is presented. The role of various types of mobile elements in distant gene transfer is considered. The nature of barriers for suc- cessful gene transfer on the level of molecular, cell and population processes is uncovered. A special attention is paid to the contribution of different systems of recombination. Hypothesis on the decisive role of horizontal gene transfer in genetic and ecological diversification of bacteria is discussed.