Group II introns are a class of retroelements capable of carrying out both self-splicing and retromobility reactions. In recent years, the number of known group II introns has increased dramatically, particularly in bacteria, and the new information is altering our understanding of these intriguing elements. Here we review the basic properties of group II introns, and summarize the differences between the organellar and bacterial introns with regard to structures, insertion patterns and inferred behaviors. We also discuss the evolution of group II introns, as they are the putative ancestors of spliceosomal introns and possibly non-LTR retroelements, and may have played an important role in the development of eukaryote genomes.