Terrorist groups must acquire funds to conduct attacks and operate their organizations. The sources of terrorist funding are divided into five categories: state support, illegal activity, legal activity, popular support, and involuntary support. For each of these different sources, there are advantages and disadvantages for terrorist groups and different opportunities for the state to utilize countermeasures to exploit different vulnerabilities. Nevertheless, targeting terrorist financing is a difficult task because the amounts raised tend to be low and easily raised and because terrorist organizations are adaptive in the face of government pressure.