Sustainability as a goal is often a major consideration in policy debates and is becoming more so in the field of evaluation. This rising profile, however, creates challenges because the contexts where sustainability is evaluated can be complex in ways that standard evaluation is less equipped to address. After addressing conditions that add to this complexity, recommendations are made to counter three problems. First, with a traditional focus on support for achieving outcomes, evaluators have tended to neglect support for managing processes. Second, with the largely accepted goal of influencing selected outcomes, evaluators have tended to neglect influencing established systems and so have been less prepared to support managing transitions from unsustainable to sustainable practices. Third, judging the value of public initiatives involves assumptions that, when applied to sustainability, can result in overly narrow and biased understandings of the public interest.