As semiconductor technology continues scaling down, the reliability issue has become much more critical than ever before. Unlike traditional hard-errors caused by permanent physical damage which can't be recovered in field, soft errors are caused by radiation or voltage/current fluctuations that lead to transient changes on internal node states, thus they can be viewed as temporary errors. However, due to the unpredictable occurrence of soft errors, it is desirable to develop soft error tolerant designs. For this reason, soft error tolerant design techniques have gained great research interest. In this paper, we will explain the soft error mechanism and then review the existing soft error tolerant design techniques with particular emphasis on SEH family because they can achieve low power consumption and small performance overhead as well.