This article argues that the evaluation of sustainability would benefit from incorporating the behaviors of complex systems into its theories, models, and methodologies. Behavior is the operative word because what matters is what complex systems do, not what complex systems are. As we apply knowledge of complexity to sustainability, it is important to recognize that sustainability is value neutral. We have values, but "sustainability" does not. Appreciation of complex behavior can help devise evaluation that will move sustainability in a direction that comports with our values. Efforts to achieve sustainability often involve multiple goals. We speak of these goals as if each can be optimized. There are powerful reasons why joint optimization is possible, but that optimization of all at the same time is not. Appreciating this reality is important if we are to devise evaluation that can support movement toward sustainability. "Emergence," "sensitive dependence," "state change," "attractors," and "self-organization" should be incorporated into evaluation designs because these complex behaviors combine to explain a great deal about sustainability. Overlaying this view with concepts from ecology and evolutionary biology provides additional complexity related insight into what sustainability is and how it can be evaluated. The material described above is brought together in a generic model that can be adapted to particular sustainability efforts.