Not every act that's a crime somewhere is a crime everywhere. That gap is exactly what the dual criminality requirement exists to police.
Put simply: most extradition treaties require that the conduct alleged against the person be a criminal offense under the laws of both the requesting country and the requested country. If what you're accused of doing isn't illegal where you're currently located, extradition generally can't proceed — at least not for that particular charge.
I say "conduct," not "offense," deliberately. This trips people up. Dual criminality doesn't require the two countries to have identically named or identically defined crimes. A court isn't going to compare statute numbers side by side and demand a perfect match. What matters is whether the underlying behavior — the actual facts of what the person allegedly did — would constitute a crime under the law of the country being asked to extradite, even if that country calls it something different or defines the elements slightly differently. Fraud in one jurisdiction might map onto a combination of "obtaining property by deception" and "false accounting" in another, and that's usually fine.
Where it gets interesting — and where I've built successful defenses — is when the conduct genuinely has no equivalent. Say a person is wanted abroad for an offense connected to currency controls, or a regulatory violation that simply doesn't exist as a criminal matter domestically. Or consider blasphemy laws, certain speech-related offenses, or some categories of "insulting a public official" that exist in various legal systems but have no criminal counterpart in, say, most Western democracies. If the requested country doesn't criminalize that conduct at all, dual criminality fails and that specific ground for extradition should fail with it.
A related wrinkle: some modern treaties and frameworks — the European Arrest Warrant is the clearest example — carve out a list of serious offense categories (terrorism, trafficking, certain financial crimes, among others) where dual criminality checking is waived entirely if the offense carries a high enough maximum sentence in the requesting state. So this protection isn't universal or absolute; its strength depends heavily on which treaty framework applies to your specific case.
It's also worth knowing that dual criminality is usually assessed at the time of the extradition request, not at the time the alleged conduct occurred, and courts generally look at whether an offense exists in substance, not whether the exact penalty ranges match.
In practice, I use dual criminality analysis early — often before a client is even arrested, if there's advance warning — because it can narrow or eliminate charges from an extradition package before the case ever reaches a hearing. It's rarely a silver bullet on its own, but paired with other defenses, it's one of the sturdier tools available.
This article is provided for general informational purposes only and does not constitute legal advice. Extradition, INTERPOL, and international criminal matters are highly fact-specific. Readers facing an actual or anticipated matter should seek advice from qualified counsel before making decisions.