Wireless Sensor Networks (WSNs) are used in many applications in military, ecological, and health-related areas. These applications often include the monitoring of sensitive information such as enemy movement on the battlefield or the location of personnel in a building. Security is therefore important in WSNs. However, WSNs suffer from many constraints, including low computation capability, small memory, limited energy resources, susceptibility to physical capture, and the use of insecure wireless comunication channels. These constraints make security in WSNs a challenge. In this article we present a study of wireless sensor network and security issues. We outline the constraints, secrity requirements, and attacks with their coresponding countermeasures in WSNs.