I'm applying to very competitive schools (Ivy League and other top engineering schools), and I'm not sure whether it's important for me to take SAT subject tests. Does the ACT just replace the SAT, or does it replace both the SAT and the SAT II?
In most cases, the ACT only replaces the regular SAT. However, there are a few schools where it also replaces SAT II requirements. These schools include:
Rensselaer Polytechnic Institute
If a college that interests you isn't on this list, you may need to send them SAT Subject Test scores in addition to your ACT scores. Check the website to verify that they ask for the SAT II in the first place. Most top colleges do at least strongly recommend that you send Subject Test results.
It's also worth mentioning that just because you can avoid sending Subject Test scores by taking the ACT instead of the SAT doesn't mean that you should. The SAT II can be a way of highlighting your expertise in a subject on top of your regular standardized test scores. If you're especially interested in chemistry, for example, you might consider taking the Chemistry SAT II to provide evidence of your skills and passion for the subject. This is something that colleges won't be able to see based on your ACT scores alone.