What is the best backpack to use when traveling across Europe?

I am going to travel across europe for a little over two months. To go "backpacking" do you have to have an actual bag that you wear on your back, or does a suitcase and daybag suffice. OR do I realy need something that can fit everything on my back?
What are some GOOD backpacks that maybe you have used, or know of others using?

I highly recommend NOT taking a suitcase/luggage with you. I did this on my first trip to Europe and it was terrible. Rolling luggage gets to be very heavy and is just really impractical for busy streets, train stations, etc... For my second trip I bought a Swiss Army Trek Pack. It has a detachable daybag that worked great. Comfortable to wear and really durable. Have a great trip!

I have been using the Kelty Coyote on my trips to europe and it has worked great. I also bring a daypack as well. You wear the big one on your back and the daypack on your front - work's great. Do not use luggage, very impractical on cobblestone.

Here's a link to the specific bag I use, but Kelty makes lots of great backpacks..

http://kelty.com/kelty/products.php?type=4&cat=66&id=33

I take a gym bag kind of thing that is my big bag, and a small backpack like a book bag for outings.
There are different brands and styles, but the most important thing I can tell you is the more waterproof, the better. If you don't have a waterproof backpack, put things like papers and clothes in plastic bags inside your pack. You'll be glad you did. I wouldn't recommend a big suitcase, but if you take even a medium sized suitcase, get one with wheels.
I have a medium sized backpack on wheels that's cool. I can stack my gym bag on top and wheel both bags at once.

If you've got money, why not take an empty bag and fill it up as you go along? Also, take old underwear and socks and throw them away! It's time for some new ones and you'll be so cool in you Euro-panties and sox.
Have fun. Don't forget the umbrella and Gore-tex (or similar)shoes and rainjacket.

If you are going for 2 months you need to wash your clothes anyhow, so you better of taking less and washing a little more often.
Whatever luggage you take, make sure you can carry it for a long while and you can lift it over your head, (on stretched arms,) to put it in an overhead rack. If your things are too heavy for that you will be very uncomfortable walking round with it.

I have been using several different backpacks, and now I use one that is as small as an on-board case for a plane, with wheels. In stations and on rougher ground I often carry it as backpack.
I try to keep it half empty when I leave home, because I always buy things like books when traveling.
For me the best amount of luggage to leave home with it 10 kilos (about 21 pounds) and to return 15 kilos (about 32 pounds) anything more I regret.
And while traveling with a big pack on your back and a heavy, smaller day pack on your front is better than many other ways, I prefer just having one pack. That allows you to carry a picnic lunch, drinks and so on too. And that small bag is going to be very heavy after a while.

I once met a girl who traveled with just one small shoulder bag, I would say about 5 kilos (10 pound) and she said it was because she was just traveling for 2 months.
I understand it now, but it takes time and a lot of effort before you can leave that many things at home.