Trouble Shooting: Backfiring

Ensure ignition timing is correct.
Ensure timing belt/chain has not jumped.
Check distributor (if applicable).
Check for gasoline in oil.
Ensure fuel injectors are not leaking.
Check spark plugs and plug wires.
Check distributor cap and rotor (if applicable).
Check ignition and coil circuit.
Check crank and cam sensors (DIS system).
Check throttle switch and connector.
Check exhaust system.