RNA world theories figure prominently in many scenarios for the origin and early evolution of life, with RNA molecules acting both as the dominant biocatalysts and as the repository of genetic information. In vitro selection has identified ribozymes that catalyze all of the major reactions of genetic information transfer and many of the chemistries associated with small molecule metabolism. A long-term goal of our program is to build new biologies by building, understanding and exploiting metabolic ribozymes. Early work examined metabolite binding by aptamers and structure-mechanism relationships in hammerhead ribozymes. Our recent focus is on the evolution and engineering of ribozymes that catalyze phosphoryl transfer (kinase and phosphatase ribozymes) for biologically relevant substrates. We are especially interested in understanding how biophysical constraints dictate the evolutionary potential of RNA.