SAT IIs are subject tests that last one-hour and are usually multiple choice.  The purpose of these tests is to evaluate your knowledge in a particular academic area and your ability to apply that knowledge.  Some colleges may recommend or even require these tests.  The 20 available tests include:

If you are taking coursework that prepares you for an AP or IB test (e.g. AP US History) and you plan to take a corresponding SAT II, be sure to take the appropriate test AS CLOSE TO THE the conclusion of your class as possible.  You will be more successful taking the test right after the completion of your class than if you attempt to remember the specific information months or years later. 

Advanced, challenging coursework can prepare you for both SAT II's and AP tests.  SAT II's may be recommended or even required for admission at some colleges.  AP tests may help you to earn college credits.  If you plan to take one of these tests, you should consider also taking the other.  See your counselor with questions.  

See also: 

SAT II Learning Center