5 Whys: repeatedly ask "why" until you reach a root cause, not just a symptom. Combine with an Ishikawa (fishbone) diagram to explore multiple causal categories simultaneously.

Example: revenue dropped -> why? conversion rate fell -> why? checkout page error rate rose -> why? a deploy broke a payment API call -> why? no integration test covered that API -> root cause: missing test coverage. The goal of RCA is to fix the systemic issue, not just the immediate symptom. Postmortems document RCA for incidents and drive process improvements, not blame.