In numerous biological processes that constitute the base of living organisms, protein function is fundamentally related to internal dynamics occurring on μs-ms time scales that can give rise to chemical exchange contributions to relaxation. In a heteronuclear two-spin system (e.g., 1H-15N), correlated motions of the two nuclei induce cross-relaxation between multiple-quantum coherences that can be quantified using new Heteronuclear Double Resonance (HDR) techniques. An analytical model describes the effect of an applied radio-frequency field on the relaxation rate of interest in the presence of fast exchange, providing accurate information on the kinetics of correlated processes. Numerical simulations and experimental results confirm the validity of the model. HDR and more conventional relaxation dispersion techniques are used to characterize the internal dynamics in several biological systems. Motions on similar time scales are detected in the two different binding surfaces in human ubiquitin and in the linker between them (Phe45), suggesting the presence of a possible global motion. Additional applications of HDR give insights into the dynamics occurring in the KID-binding domain (KIX) of the CREB-binding protein. We identified the presence of exchange processes, faster than the one reported by Konrat and coworkers, outside the main helices of KIX. These fast motions are most likely the sign of conformational disorder within the native state, which may promote the transition to the unfolded ensemble. A combination of 15N relaxation rates and magnetization transfer rates are used to provide a qualitative characterization of internal dynamics in Engrailed 2. While contribution of exchange processes in the ms time scale are small, fluctuations at sub-ms timescales are found to occur both in the unstructured part that contains important binding sites for other transcription factors, and at a few restricted locations in the structured homeodomain. Such motions are characterized by 15N R1ρ relaxation dispersion. Our results reveal that the hexapeptide motif is characterized by complex dynamics that may be linked to its physiological function. Moreover, the time scale of motions in the homeodomain is reasonably close to that of the hexapeptide region. It is therefore tempting to suppose that transient contacts occur between the structured and unstructured regions.