The advice that the previous posters put in looks quite sensible. Correct saddle height-sensible gears etc. I think that cycling is one of the least damaging exercises for the knees. It is really important I think to take care of your knees as once you have damaged them (which is easy to do) they will never be the same again. I am quite heavy (220 lbs) and running really puts my knees and lower legs under pressure. They used to get quite sore so I dont run much now. I also damaged my knees doing weight training. Even though I havent weight trained on my legs for a couple of years I sometimes still get the soreness back and this is nothing to do with cycling but is due to the previous weight training. Cycling though has never yet caused me any problems and I think that this is one of the best exercises that you can do without damaging your knees.