The development of a smarter electric grid will depend on increased deployments of information and communication technology (ICT) to support novel communication and control functions. Unfortunately, this additional dependency also expands the risk from cyber attacks. Designing systems with adequate cyber security depends heavily on the availability of representative environments, such as testbeds, where current issues and future ideas can be evaluated. This paper provides an overview of a smart grid security testbed, including the set of control, communication, and physical system components required to provide an accurate cyber-physical environment. It then identifies various testbed research applications and also identifies how various components support these applications. The PowerCyber testbed at Iowa State University is then introduced, including the architecture, applications, and novel capabilities, such as virtualization, Real Time Digital Simulators (RTDS), and ISEAGE WAN emulation. Finally, several attack scenarios are evaluated using the testbed to explore cyber-physical impacts. In particular, availability and integrity attacks are demonstrated with both isolated and coordinated approaches, these attacks are then evaluated based on the physical system's voltage and rotor angle stability.