Energy harvesting has been widely investigated as a promising method of providing power for ultra-low-power applications. Such energy sources include solar energy, radio- frequency (RF) radiation, piezoelectricity, thermal gradients, etc. However, the power supplied by these sources is highly unreliable and dependent upon ambient environment factors. Hence, it is necessary to develop specialized systems that are tolerant to this power variation, and also capable of making forward progress on the computation tasks. With the emerging memory technologies, nonvolatile processors are manufactured with nonvolatile registers and can work reliably under the environments with frequency power interrupts, such as energy harvesting and wireless powered applications. This talk first presents the motivation of nonvolatile processor design and defines the metrics for such transient computing systems. Furthermore, we will illustration the nonvolatile processor from circuit, architecture and system levels. Finally, the latest chip design efforts are reported and future research opportunities and challenges will be given out for collaborations.