Abstract Previous studies of the structural behaviour of castellated beams are reviewed and a number of different possible failure modes identified. Several of these do not occur with plain webbed beams since they are a direct result of the different way in which shear is transferred through the perforated web. Examples are a Vierendeel mechanism, web post buckling due to shear and web weld rupture. Failure by either the formation of a flexural mechanism or by lateral-torsional instability are essentially similar to the equivalent modes for solid web beams. Methods for predicting the loads at which each of these types of failure occurs are evaluated against the available experimental data and the limitations in a number of these analytical approaches is discussed. It is concluded that both lateral-torsional instability and the formation of a flexural mechanism may be handled by an adaptation of established methods for plain webbed beams, providing the cross-sectional properties are those corresponding to the centreline of a castellation. Currently available methods for the determination of collapse in the other modes, while rather less accurate, are adequate for design except in the case of web post buckling due to compression.