What makes an effective sprint retrospective?

Retrospective reflects on sprint to improve processes. Effective elements: psychological safety (blameless), focus on actionable items (specific improvements), limit action items (2-3 per sprint), assign owners, track follow-through. Formats: Start/Stop/Continue, Mad/Sad/Glad, sailboat (wind, anchors, rocks). Anti-patterns: blame individuals, no action items, same issues recurring, senior dominance. Outcomes: identified improvements, assigned actions, team bonding. Most important Scrum event for continuous improvement.