<P> On February 3 , 2016 , it was announced that Grant Gustin , who appears as Barry Allen / Flash on The Flash , would appear in the eighteenth episode of the first season , `` Worlds Finest '' . While no plot details on the episodes were released at the time , Ross A. Lincoln of Deadline.com noted that `` the in - universe reason '' for the crossover was due to Barry 's ability to travel to various dimensions , thus implying that Supergirl exists on an alternate Earth to the Arrowverse in a multiverse . The Flash episode `` Welcome to Earth - 2 '' confirmed this , showing an image of Benoist as Supergirl during a sequence where characters travel through that multiverse . The earth that the series inhabits is Earth - 38 in the Arrowverse multiverse , and has been informally referred to as `` Earth - CBS '' by Marc Guggenheim , one of the creators of Arrow . </P>
<P> On February 3 , 2016 , it was announced that Grant Gustin , who appears as Barry Allen / Flash on The Flash , would appear in the eighteenth episode of the first season , `` Worlds Finest '' . While no plot details on the episodes were released at the time , Ross A. Lincoln of Deadline.com noted that `` the in - universe reason '' for the crossover was due to Barry 's ability to travel to various dimensions , thus implying that Supergirl exists on an alternate Earth to the Arrowverse in a multiverse . The Flash episode `` Welcome to Earth - 2 '' confirmed this , showing an image of Benoist as Supergirl during a sequence where characters travel through that multiverse . The earth that the series inhabits is Earth - 38 in the Arrowverse multiverse , and has been informally referred to as `` Earth - CBS '' by Marc Guggenheim , one of the creators of Arrow . </P>
when does the flash show up in supergirl
when does the flash show up in supergirl
the eighteenth episode of the first season , `` Worlds Finest ''
the eighteenth episode of the first season , `` Worlds Finest ''