Abstract: A lightweight, free-standing, expandable shelter (10) providing protection against chemical and biological agents and against nuclear fallout. The shelter (10) has a frame comprising a series of U-shaped ribs (12) spaced and held parallel by a series of reinforcing members (16) connecting each adjacent pair of ribs (12) on each side of the shelter (10). A cover (11) made of flexible material which is resistant to chemical and biological agents is attached to the frame. An airtight floor (18) is attached to the cover material (11) so that the shelter (10) can be pressurized to a pressure of about one inch water gauge above ambient pressure, to prevent reverse air flow of contaminated air into the shelter (10). A blower (20) for pressurizing the shelter (10) and a filter (22) capable of filtering out chemical and biological agents are connected to the shelter (10).