Composite steel concrete beams are formed by connecting concrete slabs to a supporting structural steel beam. In the early 1900’s, composite beams were considered favourable for bridge design but in recent decades, composite steel and concrete structures are employed extensively in modern high rise buildings. The flexural strength of composite beams is greatly influenced by the strength and ductility of the shear connectors between the structural steel beam and the concrete slab. The behaviour of the shear connectors is important in understanding the shear force transmitted and the degree of slip which occurs at the interface of the steel and concrete. Composite steel-concrete beams are becoming increasingly popular in multistorey buildings due to their higher span/depth ratio, reduced deflections and increased stiffness value. However, their performance is highly dependent on the load-slip characteristics of the shear connectors. More recently, trapezoidal profiled slabs are becoming increasingly more popular for high-rise buildings when compared with solid slabs because they can achieve large spans with little or no propping and they require less concrete and plywood formwork. However, the profiles used to achieve these savings can have a detrimental effect on the shear connector behaviour. The issues emphasised in this thesis are: the effects of steel fibres as a strengthening system in composite steelconcrete beams: the effects of elevated temperatures on the behaviour of headed stud shear connectors for composite steel-concrete beams: the long term effects on the behaviour of the composite steel-concrete beams: the effects of strain regimes on the behaviour of the composite steel-concrete beams: the effects of the combination of axial tension and shear loading on the behaviour of composite steel-concrete beams. This thesis consists of many of numerical studies. They include the mechanical behaviour and various loading conditions of the behaviour and strength of headed stud shear connectors for composite steel-concrete beams. The finite element package known as ABAQUS was used to compare the existing experimental studies by other researchers. The outcome of the numerical analysis is very satisfying and also contains design recommendations. Furthermore, a methodical parametric study is undertaken using the model that has been properly calibrated. These parametric studies broaden the range of application for design guidance. It is fervently hoped that the findings herein are useful in contributing further insight to the continuing evolution of composite steel-concrete structures.