This document discusses the need for a phylogeny-driven genomic encyclopedia to fill gaps in knowledge of bacterial and archaeal diversity. It notes that as of 2002, genome sequences were mostly from just three phyla of bacteria, while at least 40 phyla were known to exist. Some phyla had only sparse sampling. The same trend occurred in Archaea. The document advocates using phylogenetic trees to guide genomic projects and fill in gaps, as is commonly done in some eukaryotic groups, in order to gain a more complete understanding of microbial diversity.