What kind of slacks...

It kind of depends on where in Germany you will be going. Down in the mountains of Bavaria or to cities like Berlin, Frankfurt, or Munich. The weather can be quite different in these 3 places. Could be snow, rain, windy or just plain nice and cold and sunny. Black is popular here, but if it is rainy or snowy, you will want something that won't show the dirt too much. Boots are really popular here, so perhaps pants that you can tuck into boots would be practical for you. Jeans are of course worn everywhere, except in dressy situations. Will you be sitting on trains or buses for long periods of time where you don't want to get too warm but also be comfortable, might also affect your choice.

I love to travel with cords. They are warmer than slacks, wear better, but are much more comfortable than jeans.

I would only bring long underwear if I knew I was spending a lot of time outside. On a train or bus, inside a museum, church, restaurant, department store or hotel, long underwear may be too warm. If you are hiking (not likely in Dec.), skiing (the snow isn't reliable in Dec.), or planning another outdoor activity, you will probably want them.

It does make a difference if you're in Berlin or Munich. You can google average temperatures for the cities you will visit.

Layering is still the way to go so you can be comfortable both indoors and outdoors. Cotton is still not a good material. If it gets wet, it provides almost no insulation.

Here's what I pack:

http://www.polyvore.com/cgi/set?id=1440268

For a December trip, I would probably add a knit cap and some warm gloves. Make sure your shoes are waterproof and wear a decent wool sock instead of something lighter.

I too find cords much warmer than jeans but with long underwear jeans will be fine. Take your silk underwear, knit cap & gloves and wool socks. Smart Socks are wonderful . . . expensive but well worth the price. Black pants are a must for any travel in Europe (Dockers, black denim, whatever). They can be dressed up for any occasion. I would take a pair of jeans, cords, and black trousers along with the silk underwear. Oh yes, I always take a bandanna/buff to keep the chill out around my neck. They look fantastic and keep you warmer!

Do bring the long underwear if you're going to Berlin!
I was there early November 2007, and it was cooold! I didn't bring warm enough clothes, so please don't make the mistake I did.

We will be in Munich from Dec 1-6 touring Ludwig's Castle, Dachau, then to Nurnberg (documentation facility,National Museum, shopping at the markets.etc. , then to Rothenburg(nightwatchmans walk & shopping)...I will bring black jeans..I do have some cords..We have 5 prs of smartwool socks..love them..Thank you all again

We usually go to Germany in December to do Christmas and New Years. We split our time between Wurzburg and Garmisch. I usually take stretch black velvet pants, which can be casual or dressy, and jeans. Also long underwear. Most important, don't forget gloves, a scarf, and a hat (or buy them there). Warm waterproof boots are also a key item. I have a pair of Hoegl goretex lowheeled boots that work for everything from dress to casual, even hiking. I bought them in Germany, and they weren't cheap, but they have never failed me.

Jeans are durable, but I find once they get wet they take like two days to dry in hotel,, I think cords are a great idea, as well as wool .
Take gloves for sure.

I find that alot depends on where you are from. This summer it was so hot for us in Europe, I was always melting, but we found people we toured with from Georgia and Texas found the weather not too bad,,they were used to heat. In winter are you used to weather in the 30's and 40's and rainy? It could be warmer, but, it could be cooler!

A big, warm muffler is easy to pack, and really adds to your comfort. It's also very European. I bought one last October in Amsterdam, and wore it every day!