Due to the sensitive nature of the data gathered by many wireless sensor networks (WSNs) it is becoming critical that this data be protected. However, due to the constrained nature of the resources available on sensor nodes, traditional wireless networking security solutions are not viable due to their processing requirements, power consumption, speed and communications overhead. We review the threats and attacks faced by WSNs and then the current state of the art of dedicated WSN security protocols are examined and compared, focusing on their relative strengths and weaknesses. © 2009 IEEE.