Ribonuclease H specifically cleaves the RNA strand of RNA/DNA hybrids. Based on the difference in the amino acid sequences, RNases H are classified into two major families, Type 1 and Type 2 RNases H. Type 1 and Type 2 RNases H share a common three-dimensional structure despite their poor amino acid sequence similarity. Whether their in vivo functions are cooperative or independent is of great interest. In addition, the enzymes, especially E. coli RNase HI, have been shown to be an excellent model not only for analyzing protein stability and protein folding, but also for protein engineering. I would like to summarize a recent progress in the RNase H studies.