The physics of two versions of reflective electronic papers: E-Ink™ and Gyricon™ are examined. Electrodynamic models are developed to understand their design and to optimize performance. Models are verified with analytic solutions, published data, and laboratory experiments to ensure fidelity and accuracy. Excellent agreement is obtained using a set of self-consistent parameters. Specific engineering suggestions are proposed to: lower switching voltage, reduce switching time, improve display resolution, control Gyricon charging, and create gray level images.