Offering efficient key management scheme (KMS) in WSN faces many challenges that will significantly impact the design and implementation of security protocols for WSN. The goal of KMS is to provide an effective environment in which the sensor node can communicate in a secure manner. It should be able to resolve the issue of generate, allocate the cryptographic keys in WSN in an efficient and effective manner. Hence, the methods for trustworthy allocation and management of these keys are very important for security of WSN. Many KMSs have been developed in recent years. However inherent characteristics of a WSN make incorporating security a great challenge. This paper presents a comprehensive review of current state-of-the-art of KMS designed for WSN security and compare with respect to several evaluation metrics. This paper also investigates the security requirements, goals and challenges of KMS based on existing literature reviews. We also attempt to provide insight in to potential research trends in the area of WSN security and outline the approaches that are likely to play a very important role.