Over the past few years, wireless sensor networks have gained significant attention. They have been distributed in the real world in order to collect valuable raw sensed data. Due to high density, WSNs (Wireless Sensors Networks) are exposed to faults and nasty attacks. Likewise, the sensors readings are inaccurate and unreliable, which make Wireless Sensor Networks vulnerable to outliers. Abnormal data, outliers or anomalies are usually considered to be those sensor readings that have deviated significantly from normal behavior. However, the challenge is to ensure data quality, secure monitoring and reliable detection of interesting and critical events. In this survey, we describe a comprehensive overview of existing outlier detection techniques specifically used for the wireless sensor networks. Moreover, we present a comparative table used as a guideline to select which technique is adequate for the application in terms of characteristics such as detection mode, architectural structure and correlation extraction.