The last 20 years has seen a tremendous increase in commercial and consequent scientific interest in members of the genus Bifidobacterium. Bifidobacteria are Gram-positive procaryotes that naturally inhabit the gastrointestinal tract of humans and other warm-blooded animals. Discovered at the start of the last century, bifidobacteria are considered as key commensals in human-microbe interactions, and are believed to play a pivotal role in maintaining a healthy gastrointestinal tract. Despite the generally accepted importance of bifidobacteria in gastrointestinal well-being, the underlying molecular mechanisms by which these bacteria function as probiotic commensal organisms is far from understood. Recent genome sequencing has given us a revealing insight into the genetic make-up of some members of the genus Bifidobacterium, although the availability of the full genomic sequence of complete bifidobacterial sequences represents only the first step in moving towards a better understanding of the biology of these organisms. This review will discuss the role that Bifidobacterium species play as a prominent probiotic component of our gastrointestinal microflora and provide some forthcoming insights into the general characteristics of Bifidobacterium genomes.