The management of risk has become a standard for decision making to transform the future into a calculable object. However, besides risk assessments there are many circumstances where actors are aware or have some insight that there is a lack of knowledge and that unexpected events need to be expected. This appears to be especially true for environmental decision making where growing knowledge on environmental processes opens up ever new knowledge about what is not known. Ignorance is therefore actively created alongside knowledge, and has therefore been characterized as the normal other side of knowledge: non-knowledge. This paper discusses some of the strategies used to cope with ongoing situations involving ignorance using examples from the remediation of areas containing multiple contaminant sources and plumes related to past industrial activities. Analysis of these processes of dealing with the unknown indicates that planning and policy making benefit when limits to knowledge are openly acknowledged and communicated and when scientific-technical risk assessments are calibrated with these limits in mind. Such processes can be seen as a lynchpin of successful remediation processes on contaminated land.