There are numerous wireless sensor network (WSN) applications being developed day to day. These applications range from simple environmental monitoring such as collecting temperatures in an agricultural farm to complex applications such as monitoring battle fields. As the applications increase so are the attacks. Therefore, several security protocols have been introduced to be used with the different applications which have varying security requirements; this implies that the choice for the WSNs application should be well considered. This paper discusses the wireless sensor network security requirements, the most common attacks and the most popular protocols used with WSNs. Focus is also given to the strengths and limitations of WSN security protocols to enable designers of the WSNs choose the right protocol for their applications.