Fish renal morphology has been studied since the 1960’s, but is still incomplete for most species, even for some of the most studied. The functional and morphological diversity and specialization of the fish kidney, due to environmental adaptations, makes this a large subject that remains understudied. References to studies on fish renal structure are given in Table 1. Although authors as Hentschel and Elger contributed to the knowledge of fish renal morphology in a comparative perspective, the knowledge concerning this issue is still reduced. Here we give a short comparative summary of fish renal morphology, using as baseline model the brown trout.