No One Left Behind Achievement Guide

No One Left Behind is an achievement of Mass Effect 2, awarded if your entire crew survives the suicide mission. Two things must be accomplished to earn this achievement:

A. You must save the crew of the Normandy and ensure their survival while escaping from the collector base

B. Your entire team must survive the suicide mission.

To accomplish A, you must first start the suicide mission right after your crew is abducted by the Collectors. If you don't, you arrive too late and they all die. And second, they must have an escort to lead them back to the Normandy.

To accomplish B, you'll need to have purchased the Normandy weapon, armor, and shield upgrades from Garrus, Jacob and Tali respectively; for every upgrade you do not purchase, a squad member will die during the approach to the Collector base. You will also need to assign the right people to the right specialist roles to ensure their and other squadmates survival. Also, every squad member, those in your squad in addition to those holding the collectors at bay, must be loyal or they will die. (DrBeeblebrox: A playthrough with all squadmates loyal but Jack and all specialists chosen correctly [a loyal Samara filling in as biotic specialist] resulted in Jack dying after defeating the final boss: Presumably, any non-loyal squad member dies at the end, whether in your squad or not, but I have only confirmed this with a disloyal Jack on an XBox360. [I had the same thing happen when Tali was the only one not loyal (she died).])(Really? I got No One Left Behind, and neither Jack or Zaeed were Loyal to me.)

See Mass Effect 2 Guide: Upgrading the Normandy for which upgrades you'll need and Mass Effect 2 Guide: Suicide Mission for which people are right for what role.

The Achievement is awarded after the end cinematic (this author did not get it until nearly halfway through the credits), so don't assume you didn't get it if you don't get the achievement right away. As long as you do everything right, it should be no problem.