Knee pain

[moondoggie]

check your saddle. positioning can lead to knee pain.

[indysteel]

Can you be more specific about where on you knee it hurts?

[BleeckerSt_Girl]

Two common causes of knee pain while riding can be:

Saddle either too low or too high (most often it's too low, especially with new riders). A too-low saddle typically causes pain near the front lower part of your kneecap.
or...
If you use clipless pedals, I've read that sometimes the angle at which your foot is set in can cause knee pain if it's not right for you. (I can't speak personally about this since I don't use clipless.)

[ny biker]

If you use clipless pedals, I've read that sometimes the angle at which your foot is set in can cause knee pain if it's not right for you. (I can't speak personally about this since I don't use clipless.)

This happened to me when I first got clipless pedals. The guy at the bike shop adjusted my cleats and the problem was fixed.

A few years ago I had problems with my right knee which was diagnosed as my patella tracking incorrectly. After some PT the problem is mostly gone. I learned that I tend to point my knee inward when I bend it, including when I walk down stairs. So now I make sure the knee stays over my toes when it bends.

[lunacycles]

Front of the knee pain is often a sign that your saddle is too far forward and/or your seat tube angle is too steep. This is somewhat common on small road frames utilizing 700c wheels.

[Zen]

Again, how old are you you?
Let's start from the basics.