Wireless sensor networks are composed of a large number of randomly deployed sensor nodes with limited computing ability and memory space. These characteristics give rise to much challenge to key agreement. General key agreement schemes like KDC, PKI and the Diffie-Hellman key exchange schemes are not applicable to sensor networks. Recently several key distribution schemes have been proposed specifically for sensor networks, aimed to provide high connectivity and resilience while keeping low memory usage in the sensor nodes. In this paper, we formularize and analyze these methods, and deduce general conditions for a scheme to be optimal in terms of connectivity, resilience and memory usage. The result provides guideline to design optimal schemes. Based on the result, we proposed 2 schemes that can achieve optimal connectivity and resilience with the restriction of memory space.