Low key lighting produces a high contrast image which is generally accomplished by lighting the subject so that there are very bright highlights and very dark shadows.

f/2.8, 1/60, ISO 800
Nikon D300, Nikkor 17-55 f/2.8 @ 31mm

For this to work well, you need to be able to control the light spill from your key light so that the shadows are preserved. This is easy to do in a studio, but you can also find opportunities to produce this effect just about anywhere.

I captured this image of my son playing the banjo while sitting at the piano in our family room. The only light source in the room was a small 40 watt lamp on the piano which was about two feet away from his face. This produced strong directional lighting but there was rapid light fall off because of the low wattage bulb which resulted in the deep shadows.

I waited until he glanced at his left before releasing the shutter so that I could capture the light hitting one side of his face while the other remained in the shadow which produced a dramatic effect.

It’s sometimes tempting to blast some flash to light up a dimly lit room, but look for opportunities to utilize existing light sources to produce a low-key, high contrast composition. Try to position your subject as close as you can to a lamp in a dimly lit room.  The lower the bulb wattage, the better the effect will be because of the minimal light spill into the rest of the room. You will probably need to open your aperture all the way and increase your ISO speed to compensate for the low light. Give it a try!