A bivouac shelter or bivvy (alternately bivy, bivi, bivvi) is any of a variety of improvised camp site or shelter that is usually of a temporary nature, used especially by soldiers or people engaged in backpacking, bikepacking, scouting or mountain climbing. It may often refer to sleeping in the open with a bivouac sack, but it may also refer to a shelter constructed of natural materials like a structure of branches to form a frame, which is then covered with leaves, ferns and similar material for waterproofing and duff (leaf litter) for insulation. Modern bivouacs often involve the use of one- or two-person tents but may also be without tents or full cover. In modern mountaineering the nature of the bivouac shelter will depend on the level of preparedness, in particular whether existing camping and outdoor gear may be incorporated into the shelter.