Although there are several likely instances of trans-kingdom lateral transfer of genomic sequences involving eukaryotes and prokaryotes, almost all well-documented cases of eukaryote to eukaryote transfer seem to involve mobile elements or other parasitic sequences. Consistent with general observations of phylogenetic regularity, the limited molecular evidence suggests that lateral transfer of eukaryotic genomic sequences is at best very rare. However, due to limited data, the possibility of rare transfers that could have considerable evolutionary significance cannot be ruled out. A possible propensity for lateral transfer by mobile elements may reflect their innate capacity for genomic wandering. In addition, occasional cross-species mobility may play a critical role in the long-term evolutionary survival of these elements and have been subject to natural selection. Much work is needed to fully understand the dynamics of TEs and other multigene families. Problems of paralogy, recombination, and variation in evolutionary rates currently present important difficulties in distinguishing conclusively between occasional lateral transfer and strictly vertical transfer. The importance of lateral transfers for host organisms must await answers to more general questions about the long-term evolutionary significance of mobile elements and the extent to which they can act as vectors for host genomic sequences.