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Rias are elongated inlets, arms of the sea, that penetrate in the continental zone. These forms are understood as sea flooded river valleys since the German researcher von Richthofen introduced the term into the scientific literature in 1886. Many authors have dealt with this rocky coast macroform, and most of them highlight their fluvial origin. Nevertheless, a detailed study of the different Galician rías shows the lack of fluvial deposits or forms inside them. This fact force to reconsider the contribution of fluvial and marine factors in shaping the coast, as well as the age of these forms. With regard to this last topic, many studies show the relevance of the falling sea level in rias formation during the last glacial period, but these approaches does not take into account that inside of rías there are Eemian interglacial dated forms and deposits. A careful revision on the rías'literature and the shaping processes and agents (i.e., lithology, joint degree, or the existing of forms and deposits) lead to the conclusion that rias are polygenic forms where the tectonic structure define its morphological pattern. The fluvial dynamics, initiated in the Cenozoic, triggered the valleys incision; and the marine processes were the main agent to shaping the actual character and form of this rock coast macroforms; therefore the fluvial contribution should be understood as a secondary process. [ABSTRACT FROM AUTHOR]