Transverse relaxation-optimized spectroscopy (TROSY) is based on the fact that cross-correlation relaxation rates associated with the interferences between chemical shift anisotropy and dipole-dipole interactions can be dramatically reduced. TROSY selects these slowly relaxing components of 15N-1HN or 13C-1H antiphase coherences to significantly enhanced signal sensitivity and spectral resolution for large proteins (>30 kD). The basic principles and applications of three- and four-dimensional TROSY-based triple-resonance experiments and NOESY experiments for structure-function studies of proteins are discussed in this chapter. To make applications of these experiments easier, some of the experimental setups are also described.