The smart grid further intensifies the dependence of power system operation on cyber infrastructure for control and monitoring purposes. With the advent of new-generation applications like wide-area measurement systems and demand side management, power utilities are in the process of planning/ building additional communication infrastructure to support their operational requirements. This expansion increases the effective attack surface available to an adversary and exposes the control applications that depend on it. Hence, in addition to improving cyber security of the communication network, it becomes imperative to develop control system algorithms that are both, attack resilient and tolerant. In this paper, we identify the types of cyber attacks that are directed at industrial control systems. We then identify key control loops in power systems operation and then determine the types of attacks that will be effective against each control loop. In the end, we present basic concepts of attack resilient control.