Organotins, and more specifically tributyltins (TBT), are introduced into the marine environment by paints designed to protect ship hulls against biological fouling. Lab tests have shown that bivalve reproduction is affected by TBT concentrations exceeding 20 ng⋅l−1. A dose-effect correlation scale describes the effects on embryogenesis and on larval growth: total larval mortality occurs after 12 days of exposure at a concentration of 200 ng⋅l−1, and inhibition of fertilization at 100 μg⋅l−1. At concentrations close to 1 ng⋅l−1, significant changes are observed in the sexuality of marine gastropods, reflected in an imposition of male characters in females, a phenomenon known as imposex. Imposex evolution in Nucella lapillus females includes: formation of a vas deferens, a channel between prostate and penis existing in males (phase 1), appearance and growth of a penis (phases 2 to 4), sterilization of the subject with blocking of the oviduct and accumulation of eggs within the gland (phases 5 to 6). In the final stages, females become sterile, thereby jeopardizing population renewal. These disturbances occur following exposure at TBT concentrations of 7 to 12 ng⋅l−1 approximately. Physiological and biochemical phenomena leading to imposex are still not well understood. However, there are evidences that TBT exposure tends to increase the testosterone contents in female mollusks, while progesterone and 17 E oestradiol levels remain constant. Since testosterone alone causes penis growth in the females, it is thought that imposex could be attributed to its accumulation originating from inhibition of cytochrome P450-dependent aromatase. The conversion of testosterone into 17 E oestradiol would then be inhibited by TBT. In spite of regulations banning the use of TBT as biocide in antifouling paints, current TBT contamination in coastal areas frequently reaches concentrations likely to cause imposex.