This document discusses security in wireless sensor networks. It begins with an introduction to wireless sensor networks and discusses some of their applications. It then describes common security threats and attacks against wireless sensor networks, categorizing them as active/passive and internal/external. Key security requirements for wireless sensor networks are also outlined, including confidentiality, integrity, availability, and data freshness. Finally, the document reviews various security mechanisms that have been proposed to defend against different attacks in wireless sensor networks, such as cryptography, trust management, and data partitioning.