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ZnO as an excellent candidate for UV light emitters, varistors, transparent high-power electronics, surface acoustic wave devices, piezoelectric transducers, and chemical and gas sensors could be integrated in a SiP (system-in-package). The SiP could be a critical part in sensor nodes in a sensor network. Normally, the ZnO device in SiP is fabricated with nanoscale films which can be compatible with other materials and processing in a package. However, despite the great potential for electron and photonic applications, ZnO device fabrication is difficult to obtain good ohmic contact. The low resistance and thermal stable ohmic contacts is critical to realize high-performance ZnO-based devices. In this paper, the recent advances of ohmic contacts on ZnO are analyzed and reviewed. The mechanism of the energy band bending at the interface of the semiconductor and the metal is discussed. The factors of forming good quality ZnO films such as the choice of the substrate and the method to deposit ZnO film, the effect of the contact resistance and thermal stability of ohmic contacts are summarized.