Recent years,the negative differential resistance(NDR)in nanoscale devices have attracted increasing attention.The NDR behavior has very important applications in electronic devices including switch,amplifier,and memory.NDR behavior and switch devices have been found in a number of molecular devices.In spite of a number of theoretical and experimental studies about NDR in various kinds of nanoscale devices,the origin for NDR is still under intense debate due to their structural complexity.In order to understand and modulate NDR in nanoscale devices,we give a systematic study of the mechanism of NDR in nanoscale systems and design of electronic switch devices.We find that NDR can be realized and adjusted by changing intermolecular interaction [1]; by constructing heterojunction [2]; by side groups [3],by deforming [4]; by doping [5]; by contact geometries [6]; by hybridized interface [7]; by tuning edge states [8]; by breaking symmetry [9]; and by tuning spin [10].