First, look on the box that came with your transmitter. There should be a nine-digit number there. If you don't have the box, your nine-digit number is most likely the number on the back of your clicker preceded by 000. This is so likley to be true that you can go ahead and register making this assumption. But you will have to check this assumption when you actually use your clicker in class to answer a question. Here's how to do that check. When you use you clicker to answer a question, look at the PRS screen and watch for your number. If you see exactly the six-digit number on your clicker, then your PRS ID is 000XXXXXX where XXXXXX stands for the 6-digit number on your clicker. On the other hand, if the number you see is 1XXXXXX, then your PRS ID number is 001XXXXXX If this happens to be the case for your clicker (the chances are about 1 in 100), then you will need to reregister your clicker. Thanks. GBA. P.S. There are a few transmitters with only a 4-digit number! Follow the same rules as above. Your PRS ID is either 00000XXXX or 00001XXXX You can determine which is correct by watching the screen.