Ribonuclease P, or RNase P, is the endonuclease responsible for processing the 5' end of tRNA by cleaving a precursor and leading to tRNA maturation. RNase P has been identified in all organisms and cells that need to process tRNA. The phylogenetic and biochemical analysis of P RNA has now reached the point where structural information is needed to be able to understand the wealth of information available. The long range goal of this work is to understand the structure and function of the RNA component of RNase P, through crystallographic studies. This will advance, not only our understanding of RNase P, but will also lead to a much better understanding of other large RNA molecules and will help us understand better the way RNA molecules fold and add to the emerging structural database of RNA molecules. It promises to provide new and important structural information to help fill the void in our knowledge on the structure of these important molecules.