Isolation levels control how transactions see each other's changes:
Read Uncommitted: See uncommitted changes (dirty reads). Fastest but dangerous.
Read Committed: Only see committed changes. Default for PostgreSQL.
Repeatable Read: Same query returns same results within transaction. Default for MySQL.
Serializable: Transactions execute as if sequential. Slowest but safest.
Higher isolation prevents anomalies but reduces concurrency. Choose based on your consistency requirements.