Abstract: Abstract: Selenium (Se) is an essential element for animals at appropriate levels, but could be toxic at both elevated and deficient levels. Firstly, the beneficial roles of Se in selenoproteins, antioxidants, immune systems were discussed in this review. Secondly, the effects and underlying mechanisms of Se in its toxicity to aquatic organisms such as oxidative stress, embryonic deformities and maternal transfer were also evaluated. Finally, the potential key factors influencing the dual effects of Se in aquatic organisms were summarized. In general, this study reviewed the dual effects of Se in aquatic organisms, and the controlling factors of these effects, especially fish. It is aimed to better understand the bioaccumulation, metabolism, biotransformation and the complex relationship between these processes and biological effects of Se in organisms. Further studies are warranted for the clearer understanding of the elusive and dual effects of Se, which can definitely provide valuable information for the proper use of Se.