Should I have a NHS number???

I registered with a local GP and was told that I'm set to go, but I've noticed threads on this board talking about a NHS number.  Do I need one and how do I go about getting one? 
I've already received treatment from my doctor, so I'm confused?

PS I live in Scotland, if that matters at all in regards to this.

I got my number in the mail a few weeks after registering at my uni's surgery. I didn't "sign up" for one, so I guess it's automatic when you register somewhere.

about a month after i registered with my GP here, i recieved a card with a little white sticker that showed my NHS number.  I've used it twice in 3 years.  Once to get my European health card, and once to see a doctor that wasn't my GP and who my GP didn't refer me to.  i have it filed away somewhere... why not call your GP and ask about it?

I didn't get a card (was told by my surgery that they're no longer issued) but did get a form letter from the PCT about needing a smear (they didn't have it in their records that I'd had one in June 2005) and my NHS number was on that.  Going to be registering with a new surgery soon as we've moved, hopefully giving them my number will make the transition of records smoother.

We got our NHS numbers very soon after registering.  If you registered with your GP, you should be getting a confirmation in the post very soon.