State renewable portfolio standards (RPSs) and renewable energy standards (RESs) are among the most effective devices for renewable energy development, but plaintiffs have begun to challenge the constitutionality of specific provisions of these statutes by claiming they discriminate against interstate commerce in violation of the dormant Commerce Clause. Recently, a coal interest group has brought a much broader challenge, arguing that Colorado’s RES excessively burdens interstate commerce because it purportedly discriminates against out-of-state nonrenewable energy providers. Should this attack succeed, the constitutionality of state RPSs and RESs across the nation will fall into doubt. At the forefront of these concerns is the ambitious California RPS, which mandates that utilities obtain 33% of energy from renewable sources. Other dormant Commerce Clause challenges to California environmental regulations, including the Rocky Mountain Farmers v. Goldstene litigation concerning the California Low Carbon Fuel Standard, may also influence how courts approach challenges to RPSs. This Article explores the implications that these cases and the Colorado litigation will have for the California RPS and other state RPSs, and analyzes the weaknesses of both the Colorado and California statutes. Finally, it offers a number of defenses that states can use to overcome these attacks.