NMR spectroscopy is a powerful approach for probing aspects of conformational dynamics in biological macromolecules. Methods that can be utilized to characterize dynamics on picosecond-nanosecond and on microsecond-millisecond time scales will be illustrated by applications to the cell-surface adhesion cadherin-11 molecule and the bZip domain of the yeast transcription factor GCN4. New theoretical and statistical methods for interpreting experimental spin relaxation data increase the information content and reliability of experimental results. A unifying feature of both protein model systems is that transient, sparsely populated, structures have key functional roles in molecular recognition.