In recent years, the area of Wireless Sensor Networks (WSNs) has been extremely popular in both the research community and industries. Once a WSN is deployed, sensors may generate a large volume of data during its lifetime. These large amounts of data have to be stored somewhere for future data retrieval and data analysis. The traditional way to collect data sensed by sensor nodes is that sensor nodes forward the data to a sink (or base station) immediately once they sense them. Although many WSNs operate in such an on-site mode, there are WSN applications that do not fit into the real time data collection mode. For example, data collection in Unattended WSNs (UWSNs) relies on the periodical appearance of a mobile sink. As most existing security solutions developed for traditional WSNs rely on the presence of a trusted third party, it makes them not applicable to UWSNs directly. This book contributes to security mechanisms in UWSNs considering the fact that a trusted third party does not exist in such networks.