California's Global Warming Solutions Act of 2006 limits California's greenhouse gas (GHG) emissions in 2020 to their 1990 level. Global climate change is a pressing environmental problem, and the best possible public policies will be required to address it. Therefore, analyses of prospective policies must themselves be of high quality, so that policymakers can reasonably rely on them when making the critical decisions they inevitably will face. In 2006, three studies were released indicating that California can meet its 2020 target at no net economic cost - raising questions about whether opportunities truly exist to substantially reduce emissions at no cost, or whether studies reaching such conclusions may simply severely underestimate costs. This paper provides an evaluation of these three California studies. We find that although opportunities may exist for some no-cost emission reductions, these California studies substantially underestimate the cost of meeting California's 2020 target. The studies underestimate costs by omitting important components of the costs of emission reduction efforts, and by overestimating offsetting savings that some of those efforts yield through improved energy efficiency. In some cases, the studies focus on the costs of particular actions to reduce emissions, but fail to consider the effectiveness and costs of policies that would be necessary to bring about such actions. While quantifying the full extent of the resulting cost underestimation is beyond the scope of our study, the underestimation is clearly economically significant. A few of the identified flaws individually lead to underestimation of annual costs on the order of billions of dollars. Hence, these studies do not offer reliable estimates of the cost of meeting California's 2020 target. Better analyses are needed to inform policymakers.