During the last decade, the use of probabilistic tools for assessing the reliability of structural systems under seismic excitations has attracted considerable research. This has been prompted by the damages detected in many buildings following severe earthquake events, and the fact that the uncertainties related to the structural capacity, demand and seismic loading, may be more efficiendy accounted for and quantified within a probabilistic context. With seismic probabilistic methodologies still under development, this thesis aims to explore reliability issues associated with steel Moment Resisting Frames (MRFs), and, within this context, to present appropriate tools for fragility assessment. To this end, a detailed analytical fragility methodology, is initially developed and tested through a case study, namely a u.s. code-designed, 3-storey MRF. The fragilities are presented in tenus of three FEMA-based, probabilistically defined, perfonnance levels. The developed methodology, despite being computationally intensive, involves only a limited number of assumptions, related mainly to the structural modelling, and is verified through comparisons with previously published results. Special attention is given to the in.£luence of joint failures on the seismic reliability.