Isolation levels control what concurrent transactions can see:
Read Uncommitted: See uncommitted changes. Dirty reads possible. Never use.
Read Committed: Only see committed data. Same query can return different results in same transaction.
Repeatable Read: Same query returns same results within a transaction. Phantom reads possible.
Serializable: Full isolation. Transactions execute as if serial. Slowest but safest.
Default: PostgreSQL uses Read Committed. MySQL InnoDB uses Repeatable Read.
In interviews, mention isolation levels when discussing consistency vs performance. Higher isolation = slower but safer.