Most of our uniforms have spots for internal soft knee pads. The Marines have one style, the Army and USAF (when they wear the multi-cams) have a slightly different style.
Shown are Crye pants. They have 2 setups, one with internal knee pads, one with external knee pads (seen). the Crye pants are often used by special operations forces since they are more fitting and have some better features.
I have a set with the internal knee pads and they are much more comfortable and fit better than standard uniform pants. The knee pads actually tend to stay around the knees compared with external knee pads or normal ACUs which never seem to be there when I need them.