Flashbacks
In addition to actually travel through time, events from an earlier time period in Supernatural are often explored through Flashbacks, where a character reminisces about something in their past.
Episodes
1.18 Something Wicked
Dean reminisces about an incident around 1989, when John Winchester was hunting a shtriga in Fort Douglas, Wisconsin. While John was out, Dean left Sam alone briefly and the shtriga attacked. John took the boys to stay with Pastor Jim, but when he returned he couldn't track the shtriga. Dean blames himself for its escape, and for the deaths that have occurred since.
3.08 A Very Supernatural Christmas
While the boys are on a hunt, and dealing with how to face what may be Dean's last Christmas, we flashback to 1991 when John leaves young Sam and Dean alone in a motel on Christmas Eve. Sam reveals to Dean that he has read John's Journal, and asks whether monsters are real. Dean reluctantly confirms that they are, but that their father is a hunter who kills them. Sam is upset and afraid that the monsters will come after them. Dean tries to make things better by stealing some presents for Sam. Sam responds by giving Dean an Amulet originally meant for John.
4.09 I Know What You Did Last Summer
Sam finally tells Dean what he went through while Dean was in Hell, and why he trusts Ruby so much.
4.13 After School Special
Sam and Dean return to Truman High School, Sioux City, Indiana, where they spent a month when they were teenagers.
4.21 When the Levee Breaks
Maybe not strictly speaking a flashback - as Sam is detoxifying from the demon's blood in his system, he hallucinates both his mother, and his young self.
5.16 Dark Side of the Moon
While in Heaven, Dean encounters memories of his past - setting off fireworks with a young Sam and while Sam is still a baby, comforting Mary who is upset that John has moved out. Sam remembers a Thanksgiving at the house of a schoolmate, and a time when he ran away.
6.13 Unforgiven
While Sam struggles not to remember, the audience gets an insight into what soulless Sam was like while he was hunting with Samuel Campbell.
7.03 The Girl Next Door
After he reads of a series of killings with a familiar M.O., Sam remembers a young girl who once saved his life, even though she was a monster.
7.10 Death's Door
After being shot by Dick Roman in the head, Bobby must revisit old memories to try and survive. He remembers a hunt with Rufus Turner, his childhood, the last argument he had with his wife Karen, and times with young Dean.
8.01 We Need to Talk About Kevin
Dean remembers his time in Purgatory and how he made a deal with the vampire Benny Lafitte to get out as there is a portal out humans can use. Dean agrees as long as they find Castiel first and he can join them.
Sam remembers meeting Amelia Richardson, a vet he eventually falls in love with after hitting a dog with the Impala.
8.02 What's Up, Tiger Mommy?
Throughout the episode, Dean flashes back to his time in Purgatory, remembering interrogating a monster and finally learning the location of Castiel from him. Dean and Benny find Castiel and he explains he abandoned Dean as powerful monsters, including Leviathans were after him. Dean convinces Castiel to join him and Benny in searching for the way out, but later remembers losing his grip on Castiel as Castiel calls out to him.
8.03 Heartache
Sam fondly flashes back on a picnic in a park with Amelia and Riot on his birthday.
8.05 Blood Brother
Dean flashes back on his time in Purgatory as he aids Benny Lafitte in hunting down his old nest, remembering fighting off a group of monsters with Castiel and Benny and then two Leviathans. The flashbacks show the apparent beginning of Dean's unwavering trust in Benny after he saves Castiel's life.
Sam remembers his time with Amelia, how after he got a job at the motel as a handyman he was reunited with her and she thought he was strange. However, after Riot goes into Amelia's room, they realize that they have a lot in common: both are basically drifters who have lost someone close to them.
8.06 Southern Comfort
Sam remembers how he and Amelia started a relationship and she reveals that she lost her husband Don to the war in Afghanistan. The next morning she tries to get him out of her life as she thinks he will pity her now as so many do, but Sam later explains how he lost Dean and they decide to talk about their lost loved ones.
8.07 A Little Slice of Kevin
Dean remembers finding the portal out of Purgatory and absorbing Benny's soul into his arm to bring him through it. As he and Castiel head for the portal, two Leviathans attack and they defeat them. Dean gets through the portal, but loses his grip on Castiel who is left behind as it closes.
After reuniting with Castiel, Castiel shows Dean the truth of what happened in Purgatory: Dean did not lose his grip on the angel, Castiel let go on purpose, wanting to stay in Purgatory as his penance.
8.08 Hunteri Heroici
Sam remembers how he and Amelia bought a house together and he met her father Stan Thompson. Stan is initally cold to him, but eventually warms up to him in time for them to learn that Amelia's husband Don is, in fact, alive after all.
8.09 Citizen Fang
After getting a distress text message from Amelia (in reality, a trick by Dean), Sam remembers how while drinking his sorrows away at a bar, he met Amelia's husband Don who shows himself to be a good man and suggests that they let Amelia decide which one of them she wants and they accept the choice. Sam, recognizing that Amelia deserves a chance with Don, decides to leave so that she gets that chance.
9.07 Bad Boys
After getting a call from a teenage mentor, Dean returns to a place where he lived for two months to solve a series of mysterious events. Lots of flashbacks about Dean's two months during his sixteenth year at the Boys Home.
11.08 Just My Imagination
After his childhood imaginary friend, Sully, shows up in the bunker, Sam has several flashbacks to his childhood and the day he kicked Sully out.
