Natural bacterial transformation involves the internalization and chromosomal integration of DNA and has now been documented in ~80 species. Recent advances have established that phylogenetically distant species share conserved uptake and processing proteins but differ in the inducing cues and regulatory mechanisms that are involved. In this Review, we highlight divergent and common principles that govern the transformation process in different bacteria. We discuss how this cumulative knowledge enables the prediction of new transformable species and supports the idea that the main role of internalized DNA is in the generation of genetic diversity or in chromosome repair rather than in nutrition.