Varistors (VARiable resISTORS) are electroceramic devices that exhibit a high electrical resistance which decreases dramatically at the so called breakdown voltage. This switching behavior originates from potential barriers (i.e., Double Schottky Barriers) at the materials’ grain boundaries. The barriers limit the current flow at low voltages and are lowered in an avalanche-like process at the characteristic breakdown voltage. Commercial varistors consist of specially doped, polycrystalline zinc oxide and are used for the protection of electric and electronic systems against overvoltages. In this work, the impact of the microstructure on the current-voltage characteristics of varistors is analyzed and discussed regarding an optimized device performance. Low-voltage varistors (i.e., varistors with a small number of potential barriers and consequently low breakdown voltages) have a multilayer structure with metal electrodes that are typically separated by two to three grains, i.e. one to two grain boundaries. Hence, in the extreme case, the electrical behavior of such a device is dominated by only one grain boundary. Macroscopic and microscopic compressive and tensile tests on low-voltage varistors showed, that mechanical stresses have a strong impact on the electrical behavior, leading to an increase or decrease of the resistance depending on the grain orientation in the active current paths. In addition, the tests evidenced the existence of asymmetric Double Schottky Barriers that lead to an anisotropic charge transport across the respective grain boundary. The current dominating paths in the device were detected and unveiled using a thermographic technique. Anisotropic effects like the activation of different current paths or a variable intensity of heat generation with respect to the stimulating current direction were observed. In order to find the origin of the anisotropic behavior, single grain boundaries as well as interfaces between the zinc oxide grains and the metal electrodes were examined using a micro-4-point-measurement technique. The measurements revealed, that the current-voltage curves of single grain boundaries differ regarding the nonlinearity of the breakdown region and the breakdown voltage. In some cases, a pronounced asymmetry of the single grain boundary characteristics was found. These findings can be explained by a grain orientation and polarity dependent modification of Double Schottky Barriers through piezoelectric charges that are induced by residual stresses. The 4-point-measurements also revealed a distribution of the electrical properties of grain-electrode interfaces. This observation can be explained by the existence of different Schottky Barrier heights as a consequence of the grain orientation dependent crystallographic termination of the interfaces.