The introduction of stable isotopes into proteins has significantly reduced the time requirements for structure elucidation of biomolecules. Moreover, structural studies of proteins with molecular weights exceeding the 10 kDa limit are usually not possible without uniform isotope labeling because of severe resonance overlap and inefficient coherence transfer along the rather small 3 J 1 H-1 H couplings. Nowadays, efficient expression of recombinant proteins is a prerequisite for many techniques used in structural biology, but the requirement for isotope labeling in particular often precludes NMR (nuclear magnetic resonance) structure determination of proteins isolated from natural sources. Specifically, proteins that have been uniformly labeled with 15 N and/or 13 C are commonly required for NMR spectroscopy, especially for backbone chemical shift assignment procedures, which are greatly facilitated by the use of a series of rather sensitive multi-dimensional triple-resonance NMR experiments (see Chapt. 4) [1], in a process that can also be automated with good success [2]. Moreover, the random replacement of nonexchangeable protons by deuterons reduces 1 H-1 H dipolar interactions and scalar couplings, thereby reducing peak line widths considerably and allowing structure elucidation of proteins exceeding 30 kDa [3,4]. Random fractionally deuterated protein samples also permit the use of longer mixing times in NOESY (nuclear Overhauser effect spectroscopy) experiments, since spin-diffusion pathways are largely eliminated. In addition, transverse-relaxation optimized spectroscopy (TROSY [5], see also Chapt. 10), which has been used for molecules larger than 100 kDa [6], benefits dramatically from deuteration. This stems from the fact that the TROSY component that is narrowed by the DD-CSA compensation is broadened by dipolar interactions with nearby protons.Besides uniform labeling approaches, stable isotopes can also be introduced at specific sites in proteins in order to simplify the assignment process and to isolate spectral information from the region of interest. For example, biosynthetically-directed fractional 13 C labeling offers the possibility of making stereospecific assignments of all isopropyl methyl groups of Val and Leu residues [7]. In another approach often used for solid-state NMR termed residue-specific labeling, isotope labels are introduced at single sites in a protein, as described in another chapter of this volume (Chapt. 11). A related scheme, called amino acid-type labeling, is accomplished by expression in an amino acid-based medium