The recently discovered prokaryotic adaptive immune system consists of Clustered Regularly Interspaced Palindromic Repeats (CRISPR) and the associated (Cas) genes. The CRISPR/Cas system recognizes invading nucleic acids through sequence complementarity between crRNA and foreign genetic elements and degrades the latter with the DNase and/or RNase activities of the CRISPR/Cas nucleoprotein complexes. To escape the CRISPR/Cas attack, viruses either mutate their nucleic acid sequences to reduce the sequence complementarity with the crRNA, or inactivate the CRISPR/Cas function using anti-CRISPR proteins. However, anti-CRISPR genes have been reported only for a single bacteria-phage system, and nothing is known for the archaea domain. In this presentation I will describe the identification and functional characterization of the anti-CRISPR genes encoded in archaeal viruses, and discuss the interactions between the host CRISPR/Cas system and the viral anti-CRISPR activities.