<P> Supergirl is an American superhero action - adventure drama television series developed by Ali Adler, Greg Berlanti and Andrew Kreisberg, based on the DC Comics character Supergirl (Kara Zor - El), created by Otto Binder and Al Plastino, that originally aired on CBS and premiered on October 26, 2015 . Supergirl is a costumed superheroine who is the biological cousin to Superman and one of the last surviving Kryptonians . </P> <P> The series was officially picked up on May 6, 2015, and received a full season order on November 30, 2015 . On May 12, 2016, Warner Bros. Television announced that the series had been renewed for a second season and that it would move from CBS to The CW, where it debuted on October 10, 2016 . On January 8, 2017, The CW renewed the series for a third season, which began airing on October 9, 2017 . As of December 4, 2017, 51 episodes of Supergirl have aired . </P> <Table> <Tr> <Th_colspan="2"> Season </Th> <Th_colspan="2"> Episodes </Th> <Th_colspan="3"> Originally aired </Th> <Th_colspan="2"> Nielsen ratings </Th> </Tr> <Tr> <Th> First aired </Th> <Th> Last aired </Th> <Th> Network </Th> <Th> Rank </Th> <Th> Average viewership (in millions) </Th> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 20 </Td> <Td_colspan="1"> October 26, 2015 (2015 - 10 - 26) </Td> <Td> April 18, 2016 (2016 - 04 - 18) </Td> <Td> CBS </Td> <Td> 39 </Td> <Td> 9.81 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 22 </Td> <Td_colspan="1"> October 10, 2016 (2016 - 10 - 10) </Td> <Td> May 22, 2017 (2017 - 05 - 22) </Td> <Td> The CW </Td> <Td> 129 </Td> <Td> 3.12 </Td> </Tr> <Tr> <Td> </Td> <Td_colspan="1"> </Td> <Td_colspan="2"> 23 </Td> <Td_colspan="1"> October 9, 2017 (2017 - 10 - 09) </Td> <Td> June 18, 2018 (2018 - 06 - 18) </Td> <Td> TBA </Td> <Td> TBA </Td> </Tr> </Table> <Tr> <Th_colspan="2"> Season </Th> <Th_colspan="2"> Episodes </Th> <Th_colspan="3"> Originally aired </Th> <Th_colspan="2"> Nielsen ratings </Th> </Tr>

How many episodes are there in season 1 of supergirl