Resource constraints and wireless nature of communication render the securing of wireless sensor networks (WSNs) challenging. Ad hoc deployment of sensors make key pre distribution schemes (KPSs) more suitable for establishing security associations between resource constrained sensors. Specifically, probabilistic key pre distribution schemes (PKPSs) which mandate very low computational complexity, are good choices for securing WSNs. In this thesis we examine the suitability of PKPSs for sensor networks and propose several improvements to PKPSs for their use in WSNs. The specific contributions of this thesis are fourold: ? Strategies to improve trade-offs between connectivity and collusion resistance. ? Enumeration of the need for authentication, and strategies to improve the strength of authentication. ? Exploiting imbalances between costs of different resources (computation, storage), and ? Taking advantage of external storage resources to improve security of WSNs.