If you could only bring ONE lens for a SLR camera to Europe...

The simple answer is that no zoom is "necessary". It's hard to know what to advise, since you are the only one who knows what kind of pictures you want to take. Lots of people live with whatever zoom is available on their cameras and they get along fine. I'd go with whichever one is lighter and takes up less room.

The 18-200 might be difficult to hold steady when it's extended to its long range. There's probably one with image stabilization available, but that adds to the cost.

If it was me, I'd go for the lens that was the lightest and that I expected to use frequently after the trip.

Really depends on what you will be shooting, though. If you just want to be prepared for anything from wide angle vistas to street people, the 18-70 would likely be a happy medium.

If you can, take your camera to a local shop and try a few lens before you buy.

Danielle, that's a difficult question to answer as there are a number of factors to consider. Especially for travel, I don't think there's one Lens that fits all situations. A lot depends on what sights you normally photograph? Are you mainly interested in scenery, buildings, people?

Another point to consider is what type of Camera you're using and the "crop factor". If you're using a 1.6x Camera, the 18-70 Lens will actually be 28-112. Will that provide the range you need (especially wide angle)? Given the Lenses you mentioned, I'm assuming your Camera is Canon?

While some people do minimize by carrying only one Lens, this somewhat defeats the reason for having an SLR. If only using one Lens, the argument could be made to simply take a "monster zoom" Camera rather than an SLR.

I've found that having a wide angle Lens is important for travel, as well as a "walkabout" with a good range. I use a 10-22 for WA and a 24-105 as my GP Lens.

I don't think I could travel with just one Lens.

I see I should have been a bit more specific. The camera I have is a Nikon D-80. I own a Nikon 18-70 lens, and I'm usually pretty happy with it shooting landscapes, but sometimes it frustrates me when I want to get a closer, more intimate shot of something and going to 70 doesn't work. Thus, since this trip is going to be more city-oriented, I've been debating whether I should something that has a larger range.

I think a good lens to start building a system around, and a perfect "Vacation" lens is the Tamron 18-250mm. As zoom lenses go, it's small, light, of good quality, and not too expensive ($500, maybe a little less). Less expensive yet is the Tamron 18-200mm.

http://www.tamron.com/lenses/prod/18250_diII.asp

A trouble with having only one lens is that often times you can't get far enough away from your subject and need an even wider angle lens.

Tamron also makes a good quality ultra wide angle zoom (11-18mm) you might be interested in:

http://www.tamron.com/lenses/prod/11_18mm.asp

Other than that, I think a good quality 18-70 will get the job done for you in Europe and is a good starting point for a multi lens system.

I was busy typing when you posted that you have a Nikon D80 and the 18-70mm lens. Great camera and lens!

I'd stick with what you have and either walk closer to your subject and/or "zoom" in on things with the computer later.

If you change your mind and are willing to travel with two lenses, I'd get the NIKKOR 12-24mm. If you can enlist a lens caddy, lens number three would be AF-S VR Zoom-NIKKOR 70-300mm. The "VR" is important.

Though I still think the Tamron 18-250mm is a perfect Vacation lens.

Danielle, I too own a Nikon D80. I bought mine with the 18-135 lens (28-216 when converted). My thought was that it was a good general purpose lens to begin with. I will be taking it to Germany in November. My only concern is that it is a "slow" lens (3.5-5.6). This makes indoor shots dificult. So think about a mini tripod or monopod. Going with a fixed lens at 2.8 or less would help, it can be expensive though. Keep in mind you want to travel light. While multiple lenses are great, they can be heavy. In the past I have traveled with a 28-85 lens on a Pentax flm camera. This has worked very well for me on numerous trips. I hope this has helped. Good Luck