In this thesis, shear strength models for slender concrete members without and with shear reinforcement are developed. Although well over 100 years of research have been carried out in this field, the basic failure mode of flexural shear failure is still under discussion. Until today, empirical models for shear design of members without shear reinforcement have been the common case for design practice as it was not possible to derive simple design equations from more mechanically oriented theories. On the other hand, a mechanically sound model for the design of beams with shear reinforcement was available from the very beginning with the 45°-truss analogy. In the following, more refined models with a concrete contribution or lower strut inclinations were introduced to decrease the necessary amount of stirrups. However, existing prestressed concrete bridges were often designed based on the principal tensile stress criterion and therefore built with very little shear reinforcement which was less than the currently required minimum shear reinforcement. Since the shear design check of these structures can often not be performed with current code models based on truss models with a variable strut inclination, refined models are therefore necessary for the assessment of these structures.In this thesis, a mechanical flexural shear model for beams without shear reinforcement is derived that accounts for the shear transfer actions from direct strut action, compression zone, crack processing zone, aggregate interlock and dowel action. Based on the mechanical model, a simplified closed form Critical Crack Width Model is derived. By linking the flexural shear capacity with flexural crack widths, the influence of axial forces can be accounted for consistently within this model. The comparison of the model with shear tests on RC beams, PC beams and RC beams in tension shows a very good agreement and it can be concluded that all relevant influence parameters are considered correctly.Moreover, the shear capacity of beams with shear reinforcement was investigated. The behavior of beams with 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 behave in agreement with an equilibrium based truss model with a variable strut inclination. To distinguish these failure modes in a consistent manner, a criterion based on the mechanical shear reinforcement ratio of the beam was derived. On this basis, shear design procedures for the design of new structures as well as for the economic assessment of existing structures are presented. The partial safety factors for the proposed models are determined by probabilistic evaluations according to EN 1990. This thesis thus presents a comprehensive procedure for design and assessment of structures under shear loading. Judging from test evaluations it can be expected that the presented approaches will be especially beneficiary for the assessment of existing structures like bridges.