The Scotch Gambit with 1. e4 e5 2. Nf3 Nc6 3. d4 exd4 4. Bc4 Bc5 leads to extremely interesting imbalances as white immediately sacrifices a pawn to gain a lead in development and active piece play in the center. If you respond to white with 1. e4 e5 2. Nf3 Nc6 you should be prepared to counter the Scotch Gambit as it is often employed as an effective surprise weapon due to the fact that white will also achieve activity in compensation for the pawn sacrifice early in the opening.
In the following chess video, excerpt from the Empire Chess DVD ¨How Grandmasters Refute Gambits¨, GM Victor Mikhalevski demonstrates the perils awaiting any unprepared player with the black pieces against the Scotch Gambit along with several impressive examples of successful defense.