A portable, collapsible shelter comprising: a) a rigid, hollow container comprising opposed ends, opposed vertical sides, a horizontal top and bottom and four corner rail elements, and wherein said corner rail elements each comprise outer and inner telescopic elements (110, 111), each said inner element being secured at its upper end to said horizontal top and being reversibly telescopically slidable within said outer element between lowered and extended positions; said ends, sides, top and bottom being secured to form a rigid container having the external dimensions and corner fitting locations which satisfy the standards for ISO Series 1 freight containers when said inner telescopic elements are in said lowered position; b) at least one of said vertical sides comprising a pivoting wall portion hingedly connected to said vertical side along the lower edge of said pivoting wall portion to pivot between a closed vertical position and an open horizontal position, said pivoting wall portion having an outer edge, and whereby an opening is formed in said vertical side when said pivoting wall portion is in the horizontal position; c) means associated with said container and with said pivoting wall portion for releasably securing said pivoting wall portion in said vertical position; d) means (70, 74, 76) adapted to support said pivoting wall portion for releasably maintaining said pivoting wall portion in said horizontal position; e) means for supporting a flexible cover above said pivoting wall portion while said pivoting wall portion is in said lowered horizontal position and said inner element is in the extended position, said means being removably attached to said outer edge of said pivoting wall portion and to said container when said pivoting wall portion is in said lowered horizontal position; and f) said flexible cover removably secured to said supporting means to be thereby supported above said pivoting wall portion when said pivoting wall portion is in said lowered horizontal position, thereby forming an enclosed space above said pivoting wall portion open to the interior of said container when said pivoting wall portion is lowered to the horizontal position; characterized in that: said means for supporting said flexible cover above said pivoting wall portion comprises a collapsible frame removably secured at a lower end thereof to said pivoting wall portion and removably and pivotally secured at an upper end thereof to said container and wherein said collapsible frame comprises a plurality of vertical elements which each comprise two hingedly connected portions and a locking element adapted to selectively permit relative pivoting of said two hinged portions of each said hinged vertical elements or lock said two hinged portions of said vertical elements in a rigidly aligned position. a) a rigid, hollow container comprising opposed ends, opposed vertical sides, a horizontal top and bottom and four corner rail elements, and wherein said corner rail elements each comprise outer and inner telescopic elements (110, 111), each said inner element being secured at its upper end to said horizontal top and being reversibly telescopically slidable within said outer element between lowered and extended positions; said ends, sides, top and bottom being secured to form a rigid container having the external dimensions and corner fitting locations which satisfy the standards for ISO Series 1 freight containers when said inner telescopic elements are in said lowered position; b) at least one of said vertical sides comprising a pivoting wall portion hingedly connected to said vertical side along the lower edge of said pivoting wall portion to pivot between a closed vertical position and an open horizontal position, said pivoting wall portion having an outer edge, and whereby an opening is formed in said vertical side when said pivoting wall portion is in the horizontal position; c) means associated with said container and with said pivoting wall portion for releasably securing said pivoting wall portion in said vertical position; d) means (70, 74, 76) adapted to support said pivoting wall portion for releasably maintaining said pivoting wall portion in said horizontal position; e) means for supporting a flexible cover above said pivoting wall portion while said pivoting wall portion is in said lowered horizontal position and said inner element is in the extended position, said means being removably attached to said outer edge of said pivoting wall portion and to said container when said pivoting wall portion is in said lowered horizontal position; and f) said flexible cover removably secured to said supporting means to be thereby supported above said pivoting wall portion when said pivoting wall portion is in said lowered horizontal position, thereby forming an enclosed space above said pivoting wall portion open to the interior of said container when said pivoting wall portion is lowered to the horizontal position;