Wireless Sensor Networks(WSNs) are composed of inexpensive, low powered microsensor nodes capable of sensing, processing and communicating with limited computational, low bandwidth and resources. WSNs are infra-structureless networks where the topology is based on the Type of Sink, number of hops, types of nodes and type of scheduling. The sensor nodes self-organize themselves dynamically to form a network and communications between nodes are established through broadcasting using radio signals. Research issues include connectivity, coverage, QoS, energy consumption, security and mobility of nodes. WSNs are more vulnerable to attacks. Hence, achieving security is an important research issue in WSNs. The three key issues to achieve security in WSNs are:Key Management, Secure Routing and Prevention of DoS. Two cross layer services includes Key Management and Intrusion Detection. We have achieved data confidentiality, integrity, freshness using data aggregation via data fusion, key management, Resilience against node capture, Secure Routing based on Link Strength value, intrusion detection using less number of nodes and anonymity trust management schemes.