Well the Criminal Investigative Division is exactly what it sounds like: it is the central investigative body charged with investigating and building cases against people who are charged with tax crimes. So Criminal Investigation works with a variety of different agencies to help build up the tax side of a case. For example, they can work with the FBI they can work with Homeland Security, they can work with a variety of different agencies to help build and prosecute those who cheat on their taxes. Now the reality of the situation is that the Criminal Investigation Division is a very small unit but they're a very focused unit. So the way that the government sends a message to tax criminals is with a very high conviction rate and currently the US Attorney's Office (when it comes to tax crimes) enjoys a 90 percent conviction rate. That's pretty staggering and the reason they're able to enjoy that that big of a conviction rate is because of the work that the Criminal Investigation Division does. Criminal Investigative cases often take a very long time to develop but once they do, they're very focused. They've usually got the people in their sights and they're very thorough investigations, so that's what the criminal investigative unit is, that's what it does and that's why you should be concerned about when they're involved in your matter.