We review the basic principles, structural designs, and electrical characteristics of photonic crystal waveguide modulators based on slow light Mach-Zehnder interferometers. A number of structural and electrical configurations of photonic crystal waveguide modulators are discussed. In addition to the advantages associated with the slow group velocity, photonic crystal waveguide modulators are found to exhibit other structural and optoelectronic merits for high speed modulation. We also briefly review past work on silicon modulators and present certain speed scaling of silicon modulators. The scaling is governed by certain intrinsic optoelectronic properties of silicon and fundamental nature of light, and plays a significant role in determining the minimum ac current density and power of silicon modulators as demonstrated in some recent experiments. (C) 2007 Elsevier Ltd. All rights reserved.