What are the flashback episodes of Buffy?

Answer 1

Becoming Part 1 (Flashbacks to Angel's siring, Angelus's killing of the Romani girl and subsequent cursing, Drusilla before she is sired, Angel meeting Whistler, Angel seeing Buffy for the first time, Buffy learning of her calling and killing her first vampire).

Amends (Flashbacks to several of Angelus's murders, courtesy of "The First evil."

Fool for Love (Flashbacks to Spike's killing of Xin Rong and Nikki Wood, Spike's siring--as William--by Drusilla, after his rejection by Cecily).

Selfless (Flashbacks to Anya's former life as Aud, and the events that motivated her to become a vengeance demon, including her first meeting with D'Hoffryn).

Lies My Parents Told Me (Flashback to Spike as William with his mother, before his siring, as she sings "Early One Morning.")

For more flashbacks, see the show "Angel, the Series." There are quite a few.

There are some episodes in "Buffy" which flash back to earlier episodes.

In "Doppelgangland," there are several flashbacks to the events of "The Wish."

In "Bargaining Part II," there is a flashback to "The Gift."

In "Him," there is a quick flashback of "Bewitched, Bothered, and Bewildered."

Technically, "The Gift" contains a few seconds of flashbacks to "Blood Ties," and "Intervention."

In "Weight of the World" there are some flashbacks to Buffy's "fake" memories of Dawn as a baby.