This thesis is concerned with the design development of cold-formed steel portal frames at both ambient and elevated temperatures. A full-scale site test was carried out to determine the behaviour in fire, of a portal frame structure comprised entirely from cold-formed steel. Laboratory component tests were used to investigate joint behaviour at ambient temperature. Numerical modelling was used to extend the investigations without the need for further physical testing. Good agreement between the experimental tests and non-linear finite element (NLFEA) models was obtained. The finite element models were then used in an extended study, the purpose of which was to develop the basis for a performance based design approach. At ambient temperature, the inclusion of joint stiffness in models was demonstrated as important in order to accurately predict the behaviour of frames. Design recommendations are proposed that are based on both the experimental and finite element results. At elevated temperature, the importance of base fixity and in-plane restraint from side-rails was highlighted as being crucial in preventing undesirable outwards collapse. Design recommendations in the form of a mathematical model, protection advice and construction details are proposed, that are based on both the experimental and finite element results. In the absence of published design recommendations, the guidance and NLFEA shell model presented can be used by practicing engineers to assist in design of cold-formed steel portal frames in fire boundary conditions.