Processes of decision concerning environmental processes or the impact of human activities on the natural environment often lack secure and robust knowledge. Nevertheless, such situations do not necessarily lead to uncertainties in environmental decision-making. The analysis of processes of revitalizing contaminated areas has shown that related actors explicitly deal with the unknown, i.e. with ignorance. Involved actors experiment with and find strategies dealing productively with ignorance, which aren't necessarily products of scientific knowledge. Hence, other strategies of dealing with non-knowledge need to be considered as well. Based on a dynamic concept of ignorance this paper shows how actors define the relevance of ignorance. Analyzing two revitalization projects, the paper offers an analytical model of decision, which takes into account the unknown. It also tackles issues like the legitimacy of decision-making, the understanding of failures, the preparedness for unforeseen developments as well as strategies for dealing with unexpected surprises.