Strain engineering in silicon has a long tradition for more than sixty years. Soon it turned out that the piezoresistivity of silicon, i.e. the change of resistance under strain, is ten to twenty times higher than in e.g. metals. This finding made silicon a promising alternative for strain gauges with high sensitivity. Strained silicon became also an established expedient to increase the mobility and thus the performance of silicon transistors. With the steady miniaturization of silicon structures, in the last ten to fifteen years, research activities were expanded on silicon nanowires. Atypical effects found in nanowires like anomalous- and giant piezoresistivity have the capability to improve strain sensitive sensors dramatically. While doped nanowires have been intensely investigated in the last years, the piezoresistive behavior of intrinsic silicon nanowires was mostly neglected. In this thesis the piezoresistivity of VLS grown intrinsic silicon nanowires is investigated under several measurement conditions to gain insight into the origin of the anomalous piezoresistive effect in such nanowires. Strain related electrical measurements on nanowires require an accurate and reliable mechanical characterization, which in turn requires the development of novel experimental strain engineering platforms. Such a platform was found in utilizing MEMS technology. In this work the design and manufacturing of an electrostatic actuated straining device (EASD) is presented which allows piezoresistive measurements in-situ in the scanning electron microscope, under cryogenic conditions and in the micro-Raman spectroscopy setup. Furthermore two different methods of nanowire integration into the EASD, i.e. monolithic- and "pick and place"-integration, are demonstrated. Spatial resolved noninvasive probing of the tensile strain applied to the nanowire was employed with micro-Raman spectroscopy. Due to the sophisticated design of the EASD, high strain levels can be achieved, enabling measurements of the nanowire piezoresistivity up to the fracture strain of the nanowire. Measurements under ambient conditions exhibited an anomalous piezoresistive behavior of intrinsic silicon nanowires, i.e. although the nanowire appears to be p-type, the nanowire exhibits n-type piezoresistive behavior with increasing strain. Cryogenic measurements in vacuum verified the strong dependency of the piezoresistivity on surface traps in <111> oriented silicon nanowires. This behavior is a result of a strain induced modulation of the surface potential, leading to an electron depopulation of the surface traps. The resulting change of the charge carrier majority type leads to the altered piezoresistive behavior of the nanowire. For piezoresistive measurements of the <111> oriented nanowire under 532 nm laser excitation, a piezoresistive behavior dominated by a combination of the nand p-type piezoresistive effects, was observed. Surface related effects played only a minor role. The occurring photo current exhibited a laser power and strain dependency, i.e. increasing laser power as well as strain increased the photo current in the nanowire. A Raman spectroscopy investigation of a <112> oriented nanowire revealed the existence of polytype structure domains inside the nanowire caused by periodic crystalline stacking faults. The resistivity of this nanowire was about ten times smaller compared to the resistivity of <111> oriented nanowires. The piezoresistive measurement exhibited only a weak dependency on surface related effects. Due to its versatility the EASD proved to be an excellent strain engineering platform for high strain experiments.