The Episode 19 Tradition revolves around the concept that the nineteenth episode in each season will be somewhere off the beaten track or "strange."
- The Road Not Taken (Season 1, Episode 19)- The concept of an alternate universe is first broached, although it had been hinted at for the greater part of the first season.
- Brown Betty (Season 2, Episode 19)- Considered to be the episode that began the "Episode 19" tradition. The entire episode revolves around a noir-musical themed storyline told by Walter to Ella while on a mixture of hallucinogens he created, named Brown Betty. Brown Betty is considered to be the official nineteenth episode of Season 2 (although it aired 20th because of an unaired Season 1 episode.)
- Lysergic Acid Diethylamide (Season 3, Episode 19)- An animated episode set inside Olivia's subconcious as the team tries to call her back to the surface, which was being possessed by William Bell. The concept of this episode is that the LSD induced the animated effect.
- Letters of Transit (Season 4, Episode 19)- A dystopian future plays out where Observers come back in time and take over the Earth, totally interrupting the main narrative of the season. It served as a spirngboard (and sales pitch to Fox because of low ratings) for Season Five, where the events were revealed to have been canon.
- Black Blotter (Season 5, Episode 9)- Black Blotter functioned as the fifth season's "Episode 19" because the season was an abbreviated season scheduled to contain only 13 episodes. Walter takes LSD and finds himself hallucinating people and places from his past, in addition to a fairy and other quirky hallucinations. It also included an animation sequence in the vein of Monty Python. The hallucinations served as a way to reflect on the past five seasons, including bringing back people/places of significance to the joke