With the decline in world fish stocks, our knowledge of fish reproduction has become fundamental. Reproduction is an essential commitment to future generation. It is also a continuous development process throughout ontogeny, requiring energetic, ecological, physiological, anatomical, biochemical and endrocrinological adaptations. The first chapters highlight important issues affecting fish normal ways of reproductive development; details would focus on species living in opposite environments, such as tropical and polar fishes; far related, as teleosts and cartilaginous fishes; and finally, fish having different reproductive strategies. Thereafter, since many fishes live in detrimental environments, mainly induced by the continuous input of xenobiotic substances into waterways, the authors found it highly pertinent to include this topic. Herein, the authors fix their attention on the factors and mechanisms that may well affect reproductionrelated hormonal systems as also on known consequences for fish living.