Improved understanding of the bacterial phylogenetic tree has allowed the distinction of at least 25 phyla with cultured representatives. This review surveys the diversity of cell envelope types present in these phyla and emphasises that it is important to define bacterial cell envelopes according to whether they have one (monoderm) or two (diderm) cellular membranes and, in the latter case, lipopolysaccharide as well. A comparative genomics approach, facilitated by the recent vast expansion in genome sequence information, is used here to survey the distribution of key lipopolysaccharide biosynthesis enzymes across the bacterial world and to consider alternative diderm cell envelope architectures. These data add to our understanding of microbial diversity and it is notable that the majority of phyla are likely to comprise diderm, lipopolysaccharide containing bacteria. This analysis and a critical review of the literature also suggest that members of the phylum Chloroflexi are typically monoderm.