Conformational Fluctuation is a fundamental property of
biological macromolecules like proteins in aqueous solution.  
In the past, few experimental methods yielded quantitative 
measurements of the conformational fluctuation of proteins.  
Recently, we have developed quantitative models for interpreting
experimental data from hydrogen exchange measurements on 
proteins and force measurement of single protein-ligand
complexes.  It is shown that conformational fluctuation 
can be quantitatively understood in terms of molecular
mechanics and thermodynamics.