Concrete filled tubular (CFT) columns have become increasingly used in recent decades, due to their excellent structural performance, which takes advantage of the combined effect of steel and concrete working together. This association provides a series of advantages which make CFT columns an interesting solution: high load-bearing capacity or efficient construction technology can be cited amongst their widely known benefits, which are completed with a high fire resistance without the need of external protection. Circular, square and rectangular hollow sections have been traditionally used to form these composite columns. Additionally, elliptical shapes have been recently introduced to the range of commercially available steel hollow sections. Their aesthetic appeal and reduced visual intrusion, combined with their structural advantages associated with sections of differing major and minor axis properties, make elliptical sections of great interest for designers. In this thesis, both circular and elliptical CFT columns are studied. Although the behaviour of CFT columns at room temperature has been well established in the last years, in the fire situation the degradation of the material properties gives rise to an extremely nonlinear behaviour of these composite columns, which makes it difficult to predict their failure. The state of the research in the field of the fire behaviour of CFT columns is reviewed in this thesis, showing that further investigation is needed for a complete understanding of the performance of such columns in the fire situation. In the case of the elliptical sections, this work constitutes a novel investigation. The fire behaviour of CFT columns is studied in this thesis by means of a realistic three-dimensional finite element model. The adopted values of the parameters of the model are a result of an extensive sensitivity analysis. The accuracy of the numerical model is verified against experimental results and, by means of the validated model, parametric studies are carried out in order to assess the influence of the main factors affecting their fire behaviour and to develop a simple calculation model for predicting the buckling resistance of CFT columns in the fire situation. The proposed simple calculation model is based on the general rules of Eurocode 4 Part 1.2 and has proved to provide a good estimation of the fire resistance of CFT columns as compared with real fire tests, as well as against the existing calculation methods which can be found in the design codes. The scope of this thesis is restricted to unprotected CFT columns filled with normal strength concrete and subjected to concentric axial loads.