The two criteria for flashbacks: 1. Does it serve the story? (And implied in this question is an underlying question: is a flashback the best way to serve the story?) 2. Does it serve the theme? Let's say you have a story about Character A who is pursing an external goal but is trapped in the past and can't move forward. That's a story where flashbacks just might be a stylistic choice that support the theme, provided they don't stop the momentum of the narrative. There's a reason there's such a passionate anti-flashback lobby: flashbacks wielded in a ham-handed and inappropriate way grind the story to a halt. It's easy to extrapolate from multiple bad experiences with flashbacks that "flashbacks are bad," or to give simplified advice to beginning writers that "flashbacks must be avoided". But that's just like saying "voiceovers are bad" or "don't use 'we see'". Sadly for people who want simple rules to write by, the answer with flashbacks as with all other choices is "do what works (and do it well)". I've just been having a long conversation at Zoetrope.com on this topic with a working screenwriting friend who is writing a script that depicts three similar incidents. If I'm remember right, the first is a flashback, the third takes place real time, and for the second (if he can convince his producer) a character urgently relates the incident to a second character who doesn't want to hear it. I hope he prevails over his producer (who is pushing for a second flashback), but his current strategy of three different versions strikes me as elegant, balanced, and sophisticated, especially when he uses some good techniques to make the "telling" scene be about drama and conflict and not plot-stopping exposition.