Since the discovery of the conjugation process in bacteria, many studies focusing on this issue have contributed to a better understanding of the biology, ecology, genetics and consequently to the taxonomy of bacteria. In this chapter, the mechanisms of the conjugation process in Gram-positive species were revised and detailed, including a set of events as the contact between donor and recipient cells, the DNA processing and its inter cellular transport, and the variations of the conjugal mating systems. Studies focusing on conjugative transfer in Bacillus thuringiensis, involving the detection of cry genes in large conjugative plasmids, the genetic basis of the process, the main plasmids, and methodological variations of mating systems are discussed. Nowadays conjugal mating systems are again prominence and several studies have been conducted to evaluate plasmid exchange both within and between B. thuringiensis and closely related species belonging to the Bacillus cereus group. Thus, conjugal mating systems became an important tool to understand the role of plasmids in the behavior and in genome evolution of B. thuringiensis.