In this paper, a flexible optimization-based approach to intentional islanding is presented. The decision is made of which transmission lines to switch in order to split the network while minimizing disruption, the amount of load shed, or grouping coherent generators. The approach, uniquely, considers voltages and reactive power directly when designing islands, since the optimization constraints include a piecewise linear model of AC power flow. Solution of the MILP provides islands that are balanced in real and reactive power, satisfy AC power flow laws, and have a healthy voltage profile.