Despite all our research, a truthful response to questions such as “What's going to happen to the biosphere?” will most often be “We don't know, and we won't know.” We go on to explore how, in the face of overwhelming ignorance, scientific inquiries in policy-related contexts can most responsibly and effectively be conducted. Better procedures for self-criticism and quality control in science are argued to be central to the construction of “usable ignorance.” A key role is also assigned to the design of approaches through which incomplete science can be better integrated into policy debates. © 1987, Sage Publications. All rights reserved.