Smart grid heavily relies on Information and Communications Technology (ICT) to manage the energy usage. The concept of smart grid implies the use of “smart” devices, such as smart meters or Remote Terminal Units (RTUs), that require extensive information to optimize the power grid. As the communication network is based on TCP/IP and Ethernet technology, new cyber vulnerabilities are introduced that can be exploited by malicious attackers. Cyber security has become a serious concern due to various intrusion incidents. Cyber attacks can make a significant impact on the grid, which will involve not only steady-state but also dynamic behaviors. A cyber-power system approach has been established that explicitly models the interaction between ICT and the power system. New technologies are under development to enhance the ICT vulnerability assessment and evaluate the impact of cyber attacks on system operation. This paper presents the cyber security issues in a smart grid environment and cyber attack/mitigation scenarios using a testbed at University College Dublin (UCD).