As advances in technology have enabled the development of tiny low-power devices capable of performing sensing and communication tasks, sensor networks have emerged and received the attention of many researchers. As in all distributed systems, time synchronization is an important component of a sensor network. Time synchronization in a computer network aims to provide a common timescale for local clocks of nodes in the network. Since all hardware clocks are imperfect, local clocks of nodes may drift away from each other in time, so observed time or durations of time intervals may differ for each node in the network. This article presents a survey of existing research on time synchronization in the field of wireless sensor networks. To the best of our knowledge, there is no previous work that solely surveys this specific area. The rest of this article is organized as follows. It describes how computer clocks operate and review the synchronization problem on computer networks. It also presents common sources of error/inaccuracy in synchronization systems in this section.