Publisher Summary This chapter presents inter-laboratory studies. Inter-laboratory studies play an important role in the improvement of the quality of determinations. They can be applied as method-performance studies, laboratory-performance studies or proficiency testing, and material-certification studies. The definitions of the various types of interlaboratory studies are described in the chapter. Inter-laboratory studies play an important role in the improvement of quality of chemical-trace analysis. They are one of the most flexible and efficient systems to help laboratories to improve and to demonstrate the quality of their work and may allow them to achieve accuracy. They also allow one to verify that a method is able to reach a requested quality and to certify parameters in matrix materials in wide fields of interest. The major limitation of interlaboratory studies is inherent to their organization. It needs a particular and high scientific and management knowledge.