This document summarizes a study that optimizes PID controller parameters for load frequency control in a four-area interconnected power system using particle swarm optimization. The four-area system includes thermal, thermal with reheat, hydro, and gas power plants. PID controllers are used for each area. Particle swarm optimization is used to minimize the integral of squared error performance index by optimizing the PID parameters. Simulation results in MATLAB/Simulink demonstrate improved regulation of frequency deviations and tie-line power flows with the optimized PID controllers.