CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) was discovered when I was studying molecular genetics of Escherichia coli, and it has been about 40 years since I announced it in 1987. Nowadays, CRISPR is known to everyone in the field of life sciences, but I could not imagine what it meant physiologically when I discovered it. A sequence with remarkable characteristics was found downstream of a gene involved in conversion process for alkaline phosphatase to produce its isozyme molecules, which I was studying at that time. The characteristic of a constant length of sequence including a palindromic sequence repeated at equal intervals was so impressive that I could not help but write about it in our paper. It was later found to be responsible for the adaptive immune function of prokaryotes, and the mechanism was used to develop a practical genome editing technology. This chapter summarizes what is currently known about CRISPR and the group of genes encoding the associated Cas proteins, and also provides an analysis of their evolution and phylogeny.