Worldwide, cold-formed thin-walled steel members have been widely used as primary load bearing members, such as wall studs, floor joints, columns and beams, in low to medium-rise buildings such as offices, hotels, flat blocks and houses. A potential problem of using cold-formed thin-walled steel in building structures is fire exposure. Steel has high thermal conductivity and thin-walled sections have high section factors, both leading to rapid steel temperature rises in fire. Despite their increasing use, our understanding of the performance of cold-formed thin-walled steel sections in fire has been limited, mainly to the standard fire resistance testing by manufacturers of cold-formed thin-walled steel sections. Such a practice is expensive and time-consuming and it is difficult to develop fundamental understanding of thin-walled steel structural behaviour in fire. Against this background, this thesis presents the results of a major experimental and numerical study of cold-formed thin-walled steel structures in fire. It includes the thermal performance of thin-walled steel channel sections in planar systems under fire attack from one side, the axial strength of cold-formed thin-walled channel sections and rectangular hollow sections at ambient and under high temperature conditions, and the axial strength of cold-formed thin-walled steel channel section panels under fire expose from one side. This project includes both experimental and numerical studies. The experiments include fire testing of eight small steel stud panels (300x300mm) with different types of steel section (lipped channel or unlipped channel), different numbers of gypsum boards (one layer or two layers) and either with or without interior insulation; 52 tests on short cold-formed lipped and unlipped channels at ambient and various elevated temperatures; and eight tests on full-scale steel stud panels (2200x200mm), two at ambient temperature and six under fire. The numerical studies of this project include an assessment of different design methods, ABAQUS simulation of various tests and numerical parametric studies of different systems. Modifications have been made to enable BS5950 Part 5 (BSI 1998) and ENV 1993-1-3 (CEN 2001) to predict the ultimate strength of thin-walled columns with service holes, and to extend the ability of BS5950 Part 5 (BSI 1998), ENV 1993-1-3 (CEN 2001) and AISI (1996) to deal with distortional buckling failure and the change in the strength and stiffness of steel at uniform elevated temperatures. For columns with non-uniform temperature profiles, the ambient temperature design method for cold-formed thin-walled columns in ENV 1993-1-3 (CEN 2001) is also modified to take into account the thermal bowing effects. The finite element program ABAQUS (HKS 2001) has been validated first against the test results. The validated software is used to carry out a number of parametric studies to investigate the performance of different systems, including thermal performance of steel stud and cassette systems with different numbers of gypsum boards on the exposed and unexposed sides; structural behaviour of short cold-formed thin-walled channel columns; structural performance of rectangular hollow columns of different lengths under uniform high temperatures, but with different initial imperfections; and structural performance of cold-formed thin-walled columns under non-uniform high temperatures. The results presented in this thesis should be valuable to both users and researchers of fire safety engineering and the cold-formed steel industry.