Kates et al . advocate the development of a new “sustainability science” (“Sustainability science,” Science 's Compass, Policy Forum, 27 Apr. 2001, p. [641][1]). As world development moves in unsustainable directions, it is indeed timely to expand the structure and focus of the scientific enterprise to effectively address emerging questions. The urgency for a transition to sustainability and the associated need for new directions in science had earlier been stressed by the Board on Sustainable Development of the U.S. National Academy of Sciences ([1][2]). However, although the proposed core research questions of sustainability science are apt, they are insufficient. Kates et al. list four methodological challenges: (i) spanning the range of spatial scales; (ii) accounting for temporal inertia and urgency; (iii) dealing with functional complexity and multiple stresses on human and environmental systems; and (iv) recognizing the wide range of outlooks. We would expand this list of challenges to include (v) linking themes and issues (e.g., poverty, ecosystem functions, and climate); (vi) understanding and reflecting deep uncertainty; (vii) accounting for human choice and behavior; (viii) incorporating surprise, critical thresholds, and abrupt change; (ix) effectively combining qualitative and quantitative analysis; and (x) linking with policy development and action through stakeholder participation. Sustainability science will need to transcend the determinism and incremental responses to perturbation that still dominate much research on the dynamics of combined socio-ecological systems. The evolution of methods that can adequately and rigorously capture uncertainty, the capacity for system discontinuity, and the normative content of sustainability problems defines a rich and urgent research agenda. In this regard, participatory scenario development offers one approach for systematically addressing many of the core challenges identified above. This method has been used in various contexts, such as the work of the Global Scenario Group ([2][3]), which has been used for UNEP's Global Environmental Outlook ([3][4]), and the work of the Intergovernmental Panel on Climate Change ([4][5]), and at a regional scale in the Georgia Basin Futures project ([5][6]). Such exercises have proven successful in bridging gaps between science and policy by engaging a wide range of experts and stakeholders in a systematic exploration of diverse global futures. 1. [↵][7]Our Common Journey: A Transition to Sustainability (Board on Sustainable Development, National Academy Press, Washington, DC, 1999). 2. [↵][8]1. P. Raskin 2. et al. , Great Transitions: The Promise and Lure of the Times Ahead (Stockholm Environment Institute, Stockholm, Sweden, 2002). 3. [↵][9]Global Environmental Outlook 3 (UNEP, Earthscan, Nairobi, Kenya, 2002). 4. [↵][10]1. N. Nakicenovic, 2. R. Swart , Eds. IPCC Special Report on Emissions Scenarios (Cambridge Univ. Press, Cambridge, 2000). 5. [↵][11]1. J. Tansey, 2. J. Carmichael, 3. R. Vanwynsberghe, 4. J. Robinson , Global Environ. Change , in press. # Response {#article-title-2} We thank Swart et al. for their interest in elaborating on the scientific core questions of sustainability science and the challenge of appropriate methods and approaches. We agree with many of their arguments and, indeed, addressed several of them as substantive (rather than methodological) challenges in our Policy Forum. More importantly, however, the meeting reported in our Policy Forum catalyzed a process of consultations on science, technology, and sustainability that, over the past year, has engaged more than 300 scientists and technologists from more than 40 countries in locally organized workshops on every continent except Antarctica. These consultations have reviewed the relevance of the core questions and challenges posed in our Policy Forum to the most urgent sustainability problems of specific regions. A synthesis workshop, organized in collaboration with the International Council for Science (ICSU) and the Third World Academy of Sciences in May, sought to integrate these regional perspectives and identify priority measures for harnessing science and technology in support of sustainability. The report of that meeting has been taken forward by ICSU as a contribution to the World Summit on Sustainable Development. Results of this continuing process of revision and elaboration can be found on the Forum on Science and Technology for Sustainability at [sustainabilityscience.org][12]. Each of the core questions initially raised in our Policy Forum now has a separate Web page with introductory essays, commentary, links, and resources. Emerging thinking on a number of related issues, including several of those raised by Swart et al. , is also addressed. Finally, the Forum supports a growing network of scientists and technologists interested in specific questions of science, technology, and sustainability. We invite all Science readers to peruse these discussions on the Forum, join the network, comment on any or all of the many papers and documents posted, and further the development of the virtual community of sustainability science and technology. [1]: /lookup/doi/10.1126/science.1059386 [2]: #ref-1 [3]: #ref-2 [4]: #ref-3 [5]: #ref-4 [6]: #ref-5 [7]: #xref-ref-1-1 View reference 1 in text [8]: #xref-ref-2-1 View reference 2 in text [9]: #xref-ref-3-1 View reference 3 in text [10]: #xref-ref-4-1 View reference 4 in text [11]: #xref-ref-5-1 View reference 5 in text [12]: http://sustainabilityscience.org