Adaptive immunity is acquired when foreign substances enter the body. Interestingly, this immune system exists not only higher organisms but also many bacteria. The CRISPR-Cas systems are the bacterial adaptive immune systems against foreign DNA and RNA derived from phages or plasmids. Many bacteria can memorize the sequences of foreign DNA and RNA, and remove them by means of these systems. Structural and functional studies of these systems have dramatically progressed over the past few years. In particular, one of the CRISPR-Cas systems, Cas9, has received considerable attention as a highly efficient tool for genome editing.