When the first edition of this book was published (1989), DNA sequencing was largely confined to the characterization of individual cloned genes; although some viruses had been completely sequenced, the concept of determining the complete sequence of a bacterial genome was still some way in the future. Even in the second edition (1994), the situation had only developed to the stage that ‘Genome-sequencing projects are under way for several bacterial species . . . ’ – the first complete genome sequences were to appear in 1995. In the third edition (1998), it was still possible to produce a list of the bacteria that had been completely sequenced, although with the caveat that it would be out of date very quickly. Now, it is no longer sensible even to attempt to produce such a list. The widespread use of automated, robotic methods means that new genome sequences are appearing weekly, if not daily. Coupled with this has been the advent of the polymerase chain reaction (PCR) – a ‘recent development’ in the first edition – and more recently microarrays and proteomics that exploit genome sequence data for global analysis of gene expression. This technological explosion has largely relegated many of the classical techniques in bacterial genetics to the pages of history, and poses a considerable problem in writing, or updating, a book such as this. Excluding the older methods completely would mean we would lose all sense of how the subject developed to the stage we are now at. Furthermore, the limitations of the purely molecular approach should be appreciated. Sooner or later, in order to fully understand the roles that specific genes play in the biology of an organism, one has to return to studying the organism itself, rather than just analysing its DNA sequence on a computer. So the compromise approach that we have adopted has been to slim down the description of the classical methods, while hopefully still retaining enough to provide historical perspective. This allows space to provide a fuller introduction to the world of genomics and post-genomics (the study of genome sequences and the exploitation of that data for analysis of gene expression and other features), and also to expand the discussion of the ways in which our knowledge of the biological properties of the organisms themselves has developed through the study of bacterial genetics (whether by classical or molecular methods).