The RequireComponent attribute lets automatically add required component as a dependency.

When you add a script which uses RequireComponent, the required component will automatically be added to the game object. This is useful to avoid setup errors. For example a script might require that a rigid body is always added to the same game object. Using RequireComponent this will be done automatically, thus you can never get the setup wrong.

// Mark the PlayerScript as requiring a rigidbody in the game object.
@script RequireComponent(Rigidbody)
function FixedUpdate() {
	rigidbody.AddForce(Vector3.up);
}

C# Example:

[RequireComponent (typeof (Rigidbody))]
public class PlayerScript : MonoBehaviour {
	void FixedUpdate()  {
		rigidbody.AddForce(Vector3.up);
	}
}