Sulfate-reducing bacteria (SRB) are a diverse group of prokaryotes that may be divided into four groups based on rRNA sequence analysis: Gram-negative mesophilic SRB; Gram-positive spore forming SRB; thermophilic bacterial SRB; and thermophilic archaeal SRB. In this review, we have assembled representative 16S rRNA sequences reported to date for SRB and have constructed phylogenetic trees from these sequences. Physiological characteristics particular to each of these groups are discussed, as is the availability of tested group-specific phylogenetic probes and PCR primers directed toward individual groups.