Our goal is to understand how proteins and RNA adopt and employ their functional conformations. This includes determining the driving forces, sequence of events, and relevant time scales that govern folding and function. We have developed a new method, Ψ-analysis , to quantify pathway heterogeneity in protein folding, a capability that is generally unavailable even with single-molecule methods. We also developed a method of using kinetic isotope effects to measure H-bond formation during protein folding. Our RNA folding studies, in collaboration with Tao Pan, have shown that the rate-limiting step in tertiary RNA folding - the consolidation of a metal ion binding site - occurs after the conformational search is nearly complete. We also determined that thermophilic ribozyme stability is derived from a tremendous increase in the amount of structure formed in the final folding transition. We have developed a thermodynamic and kinetic formalism for RNA folding and presented the first detailed study on RNA folding during transcription.