We have developed experimental methods for initiating nonequilibrium structural conversion processes (e.g., protein folding, peptide self-assembly, ligand/receptor complex formation, etc.) by rapid mixing and for trapping intermediate states by rapid freezing after a defined time interval, on the millisecond time scale. When combined with low-temperature dynamic nuclear polarization, selective isotopic labeling, and solid state NMR techniques, these methods allow us to characterize the time-dependence of multiple aspects of molecular structure during a rapid structural conversion process. As an example, I will describe results for the folding and self-assembly of the 26-residue peptide melittin after a rapid pH jump. The data indicate that unstructured melittin monomers at low pH adopt helical conformations and self-assemble into antiparallel dimers at neutral pH in a cooperative manner on the 6-9 ms time scale. Melittin tetramers then form quickly, but become fully structurally ordered more slowly, on the time scale of about 60 ms. The latest results from other applications will also be described. Overall, this approach to studies of nonequilibrium structural conversions has broad applicability, providing information that is not readily available in as comprehensive a manner from alternative approaches such as optical spectroscopies.