CRISPR-Cas systems constitute adaptive immune systems for antiviral defense in bacteria. We investigated the occurrence and diversity of CRISPR-Cas systems in 48 Bifidobacterium genomes to gain insights into the diversity and co-evolution of CRISPR-Cas systems within the genus and investigate CRISPR spacer content. We identified the elements necessary for the successful targeting and inference of foreign DNA in select Type II CRISPRCas systems, including the tracrRNA and target PAM sequence. Bifidobacterium species have a very high frequency of CRISPR-Cas occurrence (77%, 37 of 48). We found that many Bifidobacterium species have unusually large and diverse CRISPR-Cas systems that contain spacer sequences showing homology to foreign genetic elements like prophages. A large number of CRISPR spacers in bifidobacteria show perfect homology to prophage sequences harbored in the chromosomes of other species of Bifidobacterium, including some spacers that self-target the chromosome. A correlation was observed between strains that lacked CRISPR-Cas systems and the number of times prophages in that chromosome were targeted by other CRISPR spacers. The presence of prophage-targeting CRISPR spacers and prophage content may shed light on evolutionary processes and strain divergence. Finally, elements of Type II CRISPR-Cas systems, including the tracrRNA and crRNAs, set the stage for the development of genome editing and genetic engineering tools. © 2015 Briner et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. ; This study was supported by startup funds from North Carolina State University. The authors thank GenProbio srl for financial support of the Laboratory of Probiogenomics. DvS is a member of the Alimentary pharmabiotic Centre funded by Science Foundation Ireland (SFI), through the Irish Government’s National Development Plan (Grant number SFI/12/RC/2273). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. ; Peer Reviewed