Another word for it is "freezing." It is simply a way of controlling targeted property. Title to the blocked property remains with the target, but the exercise of powers and privileges normally associated with ownership is prohibited without authorization from OFAC. Blocking immediately imposes an across-the-board prohibition against transfers or dealings of any kind with regard to the property.

The fines for violations can be substantial. Depending on the program, criminal penalties can include fines ranging from $50,000 to $10 million and imprisonment ranging from 10 to 30 years for willful violations. Depending on the program, civil penalties range from $250,000 or twice the amount of each underlying transaction to $1,075,000 for each violation.

Office of Foreign Assets Controls, the Office within the U.S. Department of the Treasury that administers and enforces economic and trade sanctions against targeted foreign countries, terrorism-sponsoring organizations, terrorists, international narcotics traffickers and others based on U.S. foreign policy and national security goals. After Sept. 11, 2001, OFAC became a significant player in the anti-money laundering field as well.

OFAC usually has the authority by means of a specific license to permit a person or entity to engage in a transaction which otherwise would be prohibited. In some cases, however, legislation may restrict that authority

Prohibited transactions are trade or financial transactions and other dealings in which U.S. persons may not engage unless authorized by OFAC or expressly exempted by statute. Because each program is based on different foreign policy and national security goals, prohibitions may vary between programs.

OFAC administers a number of U.S. economic sanctions and embargoes that target geographic regions and governments. 

Comprehensive sanctions programs include Burma (Myanmar), Cuba, Iran, Sudan, and Syria.  Other non-comprehensive programs include the Western Balkans, Belarus, Cote d'Ivoire, Democratic Republic of the Congo, Iraq, Liberia (former regime of Charles Taylor), persons undermining the sovereignty of Lebanon or its democratic processes and institutions, Libya, North Korea, Somalia and Zimbabwe as well as other programs targeting individuals and entities located around the world. 

Those programs currently relate to foreign narcotics traffickers, foreign terrorists, transnational criminal organizations, and WMD proliferators.  It is important to note that in non-comprehensive programs, there are no broad prohibitions on dealings with countries, but only against specific named individuals and entities. 

The names are incorporated into OFAC’s list of Specially Designated Nationals and Blocked Persons ("SDN list") which includes over 6,000 names of companies and individuals who are connected with the sanctions targets.  A number of the named individuals and entities are known to move from country to country and may end up in locations where they would be least expected. 

U.S. persons are prohibited from dealing with SDNs wherever they are located, and all SDN assets are blocked. Entities owned by a person on the SDN List (defined as a direct or indirect ownership interest of 50 percent or more) are also blocked, regardless of whether that entity is separately named on the SDN List. 

Because OFAC's programs are dynamic and constantly changing, it is very important to check OFAC's website on a regular basis to ensure that your SDN list is current and you have complete information regarding current restrictions affecting countries and parties with which you plan to do business.

As part of its enforcement efforts, OFAC publishes a list of individuals and companies owned or controlled by, or acting for or on behalf of, targeted countries. It also lists individuals, groups and entities, such as terrorists and narcotics traffickers designated under programs that are not country-specific. Collectively, such individuals and companies are called "Specially Designated Nationals" or "SDNs." Their assets are blocked and U.S. persons are generally prohibited from dealing with them.