Low key imagery is dark and moody. It's usually underlit and has a strong contrast between the generally dark color of the scene, and the few areas of the subject that are lit. It is dramatic, and striking. Shooting low key imagery in a natural setting is possible, but relies on a particular set of conditions being available, notably a darker background, and a light source that is doing a good job of lighting only a portion of your subject, often from the side or back. This is more easily achieved in a controlled environment such as a studio, where a black backdrop can be created, and the light source setup deliberately. Any subject is fine for this contest, and images do not need to be in black and white, colour is fine too.