This review explores examples of horizontal genetic transfer in eukaryotes and prokaryotes. The best understood of these involves various conserved families of transposable elements, but examples of non-transposable-element-based movement of genes or gene clusters have also been identified in prokaryotic genomes. A unifying theme is the structural and DNA-sequence homology of transposable elements from widely unrelated genomes, suggesting evolutionarily conserved mechanisms for horizontal transfer. This is reinforced by the fundamental similarity in the enzymatic mechanisms of retro viral integration (by integrases) and of transposition (by transposases). The review deals with various types of horizontal transfer, the mechanisms available for such transfer, potential barriers, and the evolutionary significance of horizontal genetic transfer.