In steel-concrete composite girders, composite dowels can be used for the efficient and safe transfer of shear forces between the concrete slab and the steel section. However, important aspects of their load bearing and deformation behavior are not clarified, so far. This is where the present thesis begins.In the first step, the influence of transverse concrete cracks on the shear capacity of composite dowels was investigated. Transverse concrete cracks detach the occurring pry-out cones and reduce the shear stresses that can be transferred by aggregate interlock. In consequences, the shear capacity of composite dowels reduces in cracked concrete. For the analysis of the internal load bearing mechanisms, shear tests with different crack widths and at different crack spacing were carried out. Since well-known design approaches neglect the influence of transverse cracks, a novel mechanically based pry-out model was developed, validated by a test database and transferred to the design load level. In the next step, the load-bearing behavior of composite dowels was analyzed under tensile, compressive and combined shear and tensile loads. For the tensile and compressive capacity of single dowels and dowel groups in cracked and non-cracked concrete, design approaches were developed. These approaches are based on models for pairs of headed stud connectors on rigid anchor plates. Furthermore, an interaction model was setup for the assessment of combined tensile and shear loads.To allow for plastic design of composite beams, the shear connectors have to provide sufficient ductility and deformation capacity. The present thesis proposes models to describe the nonlinear relationship between shear load and slip, based on the shear capacity and the elastic connector stiffness. These models are used to define structural boundary conditions (such as concrete cover and dowel dimensions), that ensure a ductile deformation behavior according to EC4 (deformability> 6 mm).In the next step the nonlinear shear force- slip characteristics were implemented into non-linear FE simulations of composite beams with partial shear connection and single flange. Comprehensive parameter studies were performed in order to find the minimum degree of partial shear connection for different beam spans, dowel characteristics, dimensions of the composite cross section and loading situations. Finally, the shear force-bearing behavior of composite beams with composite dowels was analyzed at large web openings. A novel engineer model was developed, considering Vierendeel failure, shear failure of the concrete slab and failure of the connectors on the opening edges. Here, the previously developed approaches for the shear, tensile and compression behavior of composite dowels were incorporated. The model approach was validated by beam tests and converted into a practical design approach.