The reliability of bolted joints is determined by a large number of influencing factors. Self-tapping screws lead to both a reducement of process costs and an improvement of the static and cyclic strength of the connection. A large variety of thread forming screws denotes great need for research in the field of directly threaded screw connections. In terms of a systematic investigation of thread forming the installation behavior as well as the characterization of the formed nut thread will be investigated for different thread styles and surface systems. The methodological approach is based on a screening of representative thread forming screws particularly in order to evaluate the influence of the shaping point. The separated consideration of the tribological influence is investigated by varying application-typical coatings and lubricant systems for one chosen thread geometry. The torque-rotation angle curve is evaluated based on the torque gradient. There will be a systematic classification into two sections, which are characteristic for the forming process on the one hand and for the friction conditions while screwing in on the other hand. The formed nut thread shape is investigated by metallographic microsec-tions as well as by means of computed tomography (CT). Further characterization is made by the evaluation of the strain hardening effect of the formed nut thread. By means of nano-indentations hardness profiles, starting from the area of maximum deformation, are determined. With regard to practical demands, screw-in depths up to tE = 2.5xd as well as the influence of the installation speed and the size of the pre-drilled pilot hole are inves-tigated as characteristic parameters. The quasi-static load capacity is evaluated by means of pre-load measurements, stripping torque by over-driving tests and pull out tests for different depths of engagement. For the calculation of the minimum depth of engagement an analytical approach will be derived under consideration of the formed thread shape determined by the size of the pre-drilled pilot hole. The fatigue behavior is investigated comparing formed and cut threads. A systematic analysis of the influence of the load distribution on the fatigue strength is done by experimental tests with cut nut threads made from steel and aluminum for different nut heights. Due to the requirement of high reliability screw connections are increasingly installed beyond yield strength. The assessment of the pre-load level as an influence on the fatigue strength both elastically as well as over-elastically pre-loaded connections are tested in the range of 0.2 to 0.8·F0,2 as constant controlled mid-load. When an over-elastic pre-load is applied, the fatigue strength was increased up to max. 111 % . This investigation is based on an improved load distribution and compressive residual stresses, deriving from an inhomogeneous stress distribution (notch effect). In detail, further analyses are performed to evaluate the influence of the contact area and load transmitting conditions as well as for the evaluation of the surface and thread friction influence. A systematic testing by means of different ways of load application lead to an assessment of the main influencing effects on the fatigue strength. Numerical simulations on the load distribution for the tested configurations of different nut styles and materials are executed in order to validate the hypotheses derived from the experimental results and in order to expand the understanding and interpretation of their influences on the fatigue strength. Both elastic as well as elastic-plastic material behavior were mainly considered to determine notch factors and the resulting strains determined by the load distribution. The comparison of local stress-strain hysteresis for elastically stressed and over-elastically pre-loaded connections lead to an assessment of the fatigue behavior and the evaluation of the mid-load dependency of bolted connections.