The present chapter aims to be a literature review on the brackish water mysid Neomysis integer, with focus on its feeding ecology, life history aspects, behaviour, physiology, biochemical composition, bioenergetics and ecotoxico10gy. All records on the species, available from literature, are listed as an appendix. The review aims to identify the state-of-the-art and the gaps in our knowledge on the species. Abundant information is available on the distribution patterns of Neomysis integer in enclosed brackish waters and estuaries, although on has to keep in mind that the swarming behaviour, vertical and horizontal migration, segregation of life stages and escape behaviour of N. integer (all not fully understood yet) can handicap the quantification of the density and biomass, and may hamper the calculation of the production and the study of the life history of the species. There is a great need for the description of the feeding ecology of key species - like Neomysis integer - in estuarine environments for the development of accurate C-flux models and the description of detritus based food web patterns, including the quantification of transfer coefficients. Although N. integer is described as an important food item for many demersa1 and pelagic fish, larger epibenthic crustaceans and wading bird species, quantitative information is still lacking on its own diet, feeding rates, feeding patterns and selectivity (especially for popu1ations living in estuarine conditions). Numerous data are available on the life history of Neomysis integer over a wide geographical and habitat range, although southern popu1ations < 51°N) are more poorly known. Variations are observed between these popu1ations in the number of cohorts, size-at-maturity, fecundity and growth rate. Growth and reproduction are affected by prevailing environmental conditions as generally observed in Crustacea. However, in the eurythermic and euryhaline Neomysis integer, typically living in the highly dynamic estuarine environment, this is not thoroughly studied yet. Details on how intra- and post-marsupial development, moulting processes and reproduction are affected by a wide range in salinity, temperature, food quantity and quality are still lacking. The biochemical composition and the ecophysiology of Neomysis integer are well known and several methodologies to calculate the energy budget have been applied to the species. There has been an increasing interest in using the brackish water mysid Neomysis integer as a toxicological test species for Western European estuarine systems. Mortality, respiration, swimming behaviour, testosterone metabolism and energy budgets are well established endpoints for bioassays with the species. However, more data on its growth, moulting and development are needed (at the individual- and population-level). The influence of prevailing environmental variables on these processes, as well as their optimal range have to be known in order to develop optimal laboratory cultures and to differentiate between chemically induced variability and natural variability in toxicity testing. Because Neomysis integer is often used as a model species to study the ecology of brackish water crustaceans, and because the species is easy to sample qualitatively in shallow water and easy to keep in the laboratory, many studies and data are available concerning the species. The older studies (< 1980) are often superficial or based on a limited number of observations; many data are often unpublished (as Ph.D. theses or reports) or published in local journals. Information extracted from this 'grey' literature has been integrated in the review. In the last 10 years, more in-depth studies have been published concerning aspects of the feeding ecology, population dynamics, physiology, bioenergetics, behaviour, and ecotoxicological use of the species, as well as on molecular work. One should consider this chapter as a reference work, to refer to when specific information on one of the topics is needed. Reading of the review is not necessary to understand the context of the following chapters. Since its content differs from the remainder of the thesis, the chapter has a separate reference list and font. It has a separate table of contents to help to navigate through the text. The author's contributions (results presented in this thesis and some unpublished work) are highlighted in the text.