Who is responsible for stopping counterfeit currency?

The government is responsible for designing currency to be very expensive and difficult to counterfeit, and law enforcement is responsible for tracking down, stopping & bringing to justice counterfeiters.

But individuals & businesses (you!) are responsible for not accepting counterfeit currency from any other person or business. And if you to accept some counterfeit currency that somebody passed to you, is your responsibility to notify law-enforcement & cooperate in the investigation.