Introduction : "The occurrence of pathogenic microorganisms in seawater or in shellfish could exist anytime sewage from human or animal origin would be discharged to the coast" (Metcalf, 1982). According to the diseases occurring in the human population or in animals, pathogens might be present in recreational waters or in shellfish. Thus, the presence of human enteric viruses (norovirus, astrovirus, rotavirus, hepatitisAvirus (HAV)) and pathogenic bacteria (Salmonella, Listeria monocytogenes, Shiga-toxin-producing Escherichia coli (STEC), Vibrio cholerae, Vibrio parahaemolyticus, etc.) has been reported in coastal areas for a long time (Colwell, 1978; Metcalf, 1978; Melnick et al., 1979; Grimes, 1991; Bosch et al., 2001;Kong et al., 2002). These microorganisms have been implicated in gastrointestinal and respiratory illnesses and other infections (skin, eyes, etc.), (Griffin et al., 2003). Using risk-assessment models for viruses, maximum risks were estimated to be 1.3 infections per 100 swimmers (Colwell et al., 1996). To evaluate the risk due to the presence of these pathogens in the environment, certain criteria have to be determined. Among them, the infectious dosewould be of a greatest importance (Table 14.1). Even if the infectious dose vary with the strains, the age of the patient, or other parameters, some pathogens are highly dangerous for men even at low concentrations (HAV, E. coli O157:H7, V. cholerae), whereas others have to be ingested in high concentrations to be harmful (V. parahaemolyticus) or are highly infectious but not very dangerous (norovirus). Thus, for some pathogens, a low contamination in seafood, for example, is not acceptable, based on risk-assessment models (Colwell et al., 1996). Among bacteria, the Vibrio family plays an important role in infections, waterborne or seafood diseases, especially in countries surrounded by warm marine waters. Toxigenic V. cholerae O1 and O139 are the causative agents of cholera in developing countries, whereas, non-O1 and non-O139 strains may also be pathogenic but are rarely associated with epidemics (Anonymous, 2002). V. parahaemolyticus has been recognized as a major cause of foodborne gastroenteritis in Japan and Eastern countries and has been linked to seafood consumption. In the United States, more than 700 cases of illness due to V. parahaemolyticus and associated to raw oyster consumption were reported between 1997 and 1998. However, only a few outbreaks have been reported in Australia (1990-1992) and France (2001). The difference between the countries could be attributed to the ecology of this bacteria that requires elevated temperature to grow (>18?C). Vibrio vulnificus associated with primary septicemia, has also been detected in a variety of raw seafood products in Korea and Japan (Anonymous, 2002). Concerning enteric bacteria discharged into marine waters, bacterial control of seafood lowered the risk of Salmonella outbreaks. The occurrence of other bacteria such as Campylobacter jejuni and L. monocytogenes has rarely been reported in shellfish (Feldhusen, 2000; Montfort et al., 1998; Dupray et al., 1999; Federighi et al., 1998). However, L. monocytogenes have been implicated with smoked fish consumption (Rocourt et al., 2000). Human enteric viruses are involved in more than 30% of the outbreaks linked to oyster consumption. Despite the variety of viruses that can communicate shellfish only, HAV and norovirus have been epidemiologically linked to disease (Lees, 2000; Le Guyader et al., 2003). The objective of this review is to gather information on the input and behavior of the main enteric microorganisms-Vibrio family excepted-via the water route and the factors able to affect the seawater and seafood quality.