[en] The primary goal of the IAEA Environment Laboratories (NAEL) is to help Member States understand, monitor and protect the marine environment. The major impact exerted by large coastal cities on marine ecosystems is therefore of great concern to the IAEA, particularly to its Environment Laboratories. The marine pollution assessments needed to understand such impacts depend on accurate knowledge of contaminant concentrations in various environmental compartments. Two fundamental requirements to ensure the reliability of analytical results are quality control (QC) and quality assurance (QA). Since the early 1970s, NAEL has been assisting national laboratories and regional laboratory networks through its reference material programme for the analysis of radionuclides, trace elements and organic compounds in marine samples. Relevant activities include global interlaboratory comparison exercises and regional proficiency tests, the production of marine reference materials, and the development of reference methods for analysis of trace elements and organic pollutants in marine samples. QA, QC and associated good laboratory practice should be essential components of all marine environmental monitoring. QC procedures are commonly based on the analysis of reference materials to assess reproducibility and measurement bias. QA can be realized by participation in externally organized laboratory performance studies, also known as interlaboratory comparison exercises, which compare and evaluate the analytical performance and measurement capabilities of participating laboratories. The need for good QA/QC in the chemical analysis of marine environmental samples is widely recognized and has been tested in a number of international QA exercises. Such diligence also needs to be applied to other components of the monitoring exercise, since these may represent a greater source of error in many instances. Data that are not based on adequate QA/QC can be erroneous, and their misuse can lead to poor environmental management decisions. The IAEA has a long history of organizing interlaboratory studies, which have evolved to include an increasing array of potential contaminants in the marine environment. A marine certified reference material (CRM), IAEA-452, prepared with a scallop (Pecten maximus) sample, was recently produced by the IAEA and certified for trace elements and methylmercury (MeHg). This species of scallop is a common, widely consumed seafood that is also used as a bioindicator for trace metal contamination in marine pollution studies. This publication presents the sample preparation methodology, material homogeneity and stability studies, evaluation of certification campaign results, and assignment of property values and their associated uncertainty. The reference values and associated expanded uncertainty for nine trace elements (As, Cd, Cr, Cu, Fe, Hg, Mn, Pb and Zn) and MeHg in the scallop sample are established. The informative value for one more element (Ni) is also given. The new CRM can be used for the development and validation of analytical methods in the determination of trace elements and MeHg in seafood Pecten maximus as well as for QA/QC purposes