Wireless Sensor Networks have been increasing in Size and complexity .WSNs have numerous applications including monitoring of oceans to monitoring performance of machinery , monitoring saftey of some area to monotoring a whole earhquake affected area and monitoring criticial military area to battelfield. Without security, the use of WSNs in any application domain would result in undesirable consequences.This book contains the detail explanation of one of the routing attack (Selective Forward Attack )in Wireless Sensor Network.This attack will occurs in the network layer . Many researchers proposes defensive mechanisums to solve this problem. Cumulative acknowledegement based and energy based is proposed and discussed . This books gives an overall idea to evaulate the detection schemes and to motivate the readers to solve the other routing attacks in wireless sensor network. Sepcial thanks to the faculty Colleague and students who gave their suggestions and performed excellent work in simulating the schemes.Finally I wish to thank Tatiana Melnic,Acquisition Editor for his encouragement and bring this work in the form of monagraph.