Sustainability science is being developed in constructive tension between a descriptive-analytical and a transformational mode. The first is concerned with analyzing problems in coupled human-environment systems, whereas the second conducts research on practical solutions to those problems. Transformational sustainability research is confronted with the challenges of generating actionable knowledge, incorporating knowledge from outside academia, and dealing with different values and political interests. This study approaches the theory and promise of sustainability science through a comparative appraisal of five empirical sustainability science projects. We exemplarily appraise in how far sustainability science succeeds and fails in yielding solution options for sustainability problems based on an evaluative framework (that accounts for the particularities of sustainability science). The selected sustainability projects cover a range of topics (water, bioenergy, land use, solar energy, urban development), regions (from coastal to mountainous, from rural to urban areas, in several countries in Africa, Europe, and South and North America), spatial levels (from village to country levels), and research approaches. The comparative results indicate accomplishments regarding problem focus and basic transformational research methodology, but also highlight deficits regarding stakeholder participation, actionable results, and larger impacts. We conclude with suggestions on how to fully realize the potential of sustainability science as a solution-oriented endeavor, including advanced collaborative research settings, advances in transformational research methodologies, cross-case generalization, as well as reducing institutional barriers.