The SAT subject tests, also known as ‘SAT IIs’, are subject-specific tests. First, the basics: each is scored on a scale of 200 to 800, consists completely of multiple choice questions, and lasts an hour. They are offered six times per year. At any given test date, you are able to register for up to three SAT IIs, back-to-back. When to take the tests and how many to take at a time is a strategic conversation in itself--one that we will save for another day. The CollegeBoard offers 20 subject tests in 5 general subject areas: mathematics, English, history, languages, and science.
Whether or not they are ‘really’ necessary is up to you and your college application goals! There are two main circumstances in which you should complete the SAT subject tests:
First, if the college you are applying to requires or recommends taking them, then the Subject Tests are a must. In many cases, specified programs for a particular subject matter will require the applicant to submit a subject test directly relevant to it. Other schools require a certain number of subject tests in general. For example, Georgetown requires any three subject tests.
The second circumstance would be to show that you excel in a particular subject matter. Yes, the admissions officers will have your GPA and transcript. But SAT IIs provide the unique opportunity to show your ability to master specific subject matter on a standardized level. Say, you are strong at Spanish. Yes, it is excellent to ace your AP exam and get a 5 and to have an A+ for eight semesters in a row. Where the SAT II comes in is showing how strong you are at Spanish on a national scale, by giving percentiles and a more detailed score out of 800. In this regard, of showing subject-specific achievement, SAT IIs can only strengthen your application.
Additionally, you should consider the fact that unlike the SAT or ACT, these tests pertain directly to a curriculum. For example, if you are a sophomore in an Honors or AP Chemistry class containing SAT II Chemistry curriculum, it's a good idea for you to sit for the SAT II and see how you perform. Even if you have not begun your college search, this will save you the hassle of re-studying for the exam in the future (say, at the end of your junior year), when instead you could be focusing on the SAT itself.
It is smart to speak to a guidance counselor, tutor, or mentor before taking the SAT subject tests. They can provide light on what is the best choice for you, and how to create a killer strategic plan that uses SAT II's to your advantage. A special note: you should proceed with caution, as these tests might appear on your complete testing history, and you might not have a choice in whether or not you can withhold your scores from a college. Feel free to bring this up with your tutor at your next session!