Entity search differs from traditional search engines in that the results of traditional search engines are Web pages, whereas the results of entity search are entities which can enhance the user's search experience. Entity search can be further categorized into the task of related entity search and the task of similar entity search. In this paper, a survey is presented on the techniques of entity search. Firstly, entity search is defined formally, and frequently used evaluation measures are introduced as well. Secondly, the algorithms of the two different types of entity search on two different data sources (unstructured data and structured data) are reviewed in details. Finally, open research issues and possible future research directions are discussed.