<P> The first indication of potential crossover events in the Arrowverse occurred on The CW during the 2013 -- 14 television season , when Barry Allen was introduced in the eighth episode of Arrow 's second season ahead of the debut of The Flash . In July 2014 , it was announced that the eighth episodes of the third season of Arrow and the first season of The Flash would be a two - hour crossover event . The crossover was originally planned as the seventh episodes of each series , but was pushed back due to the large amount of work needed to accomplish it . In particular , the schedule coordination of trying to `` jam another episode into the 23 - episode schedule for each of ( the ) shows . '' Marc Guggenheim , creator and executive producer on Arrow , explained that there was `` no financially responsible '' way of executing the crossover , with budgets being blown , long hours and the actors having to film scenes from both series on the same day . </P>
when did the flash first appear on arrow
in the eighth episode of Arrow 's second season