A smart grid requires the implementation of ICT technologies in order to incorporate new functions into electricity grid monitoring and control. Wide Area Monitoring Systems (WAMSs) are used to measure synchrophasor data at different locations and give operators a near-real-time picture of what is happening in the system. The measurement data is periodically collected via communication channels to monitor, predict and control the power consumption, and detect any problems in the power grid. Attacks on WAMSs can trigger wrong decisions and create dangerous failures in the smart grid system. In this paper, we investigate data integrity attacks at different attack entry points of a WAMS, their impacts on the smart grid system, and existing mitigation strategies. We conclude from our study that the existing techniques, methodologies and mechanisms are not effective enough to detect or mitigate some attacks.