Greek as in GRΣΣK the television series.
The Greek theme song is not a stereotypical theme in that it is not played in a opening-sequence manner. The song, however, is widely accepted as the show’s real theme as it was played all throughout commercials and trailers for the series’ first season.
The song in question is called “Our Time Now” by the band Plain White T’s. It was originally from their fourth album Every Second Counts; the song is the third single from said album. It was also performed by the band in an episode of Greek.
It is also known that subsequent promos for the show used Plain White T’s music exclusively.
Below is their appearance on the show performing “Our Time Now”.