In bacteria, transformation is the alteration of a cell by the uptake and expression of exogenous DNA. It can occur naturally or be artifically induced in the laboratory. Bacteria who are capable of transformation, either naturally or artificially, are said to be competent. A bacterium whose genetic constitution has been altered by transformation is called a transformant.

Conjugation and transduction are other genetic processes that may transfer DNA between bacteria. The eukaryotic equivalent of transformation is called transfection. Transformation is also sometimes used to refer to the artificial insertion of genetic material into plant or animal cells.

Natural competence is the term referring to bacteria who can take up exogenous DNA by natural means. About 1% of bacterial species are naturally competent in the lab, and many more show natural competence in their natural environments. Natural competence occurs because some bacteria carry genes that express the proteins needed to transfer DNA across cell walls and membranes.

Artificial competence is the ability of bacteria to be transformed as a result of laboratory processes that make the bacterium permeable to DNA by exposing it to non-natural conditions.

Calcium chloride transformation is one method used in the lab to produce artificial competence. The cells are chilled in the presence of divalent cations such as Ca2+ which (as in CaCl2) which prepares the membrane to be permeable to plasmid DNA. Then the cells are briefly heat-shocked (42C for 30-100 seconds) allowing the DNA to enter the cells. This method works very well for circular DNA, but not so well for linear DNA (such as chromosome fragments), probably because the linear DNA can be degraded by the cell's own exonuclease enzymes. Interestingly however, naturally competent bacteria transform linear DNA more efficiently than circular DNA. Another method, called electroporation, involves exposing the cells to a brief shock of 10-20kV/cm which creates holes in the membrane for DNA uptake. This is particular useful when transforming bacteria with large plasmids, for example. The holes are then swiftly repaired by the cell's membrane-repair mechanisms.