In this thesis, a mechanical flexural shear model for beams without shear reinforcement is derived that accountsfor the shear transfer actions from direct strut action, compression zone, crack processing zone, aggregateinterlock and dowel action. Based on the mechanical model, a simplified closed form Critical CrackWidth Model is derived. By linking the flexural shear capacity with flexural crack widths, the influence of axialforces can be accounted for consistently within this model. The comparison of the model with shear tests onRC beams, PC beams and RC beams in tension shows a very good agreement and it can be concluded thatall relevant influence parameters are considered correctly.Moreover, the shear capacity of beams with shear reinforcement was investigated. The behavior of beamswith very little shear reinforcement can be considered similar to the behavior of beams without shear reinforcement,but with an additional stirrup contribution. For higher shear reinforcement ratios, the beams behavein agreement with an equilibrium based truss model with a variable strut inclination. To distinguish thesefailure modes in a consistent manner, a criterion based on the mechanical shear reinforcement ratio ofthe beam was derived. On this basis, shear design procedures for the design of new structures as well as forthe economic assessment of existing structures are presented. The partial safety factors for the proposedmodels are determined by probabilistic evaluations according to EN 1990. This thesis thus presents a comprehensiveprocedure for design and assessment of structures under shear loading. Judging from test evaluationsit can be expected that the presented approaches will be especially beneficiary for the assessment ofexisting structures like bridges.