In the ICU, scientific uncertainty about prognosis represents the most common type of uncertainty that complicates communications between providers and families. In managing this form of uncertainty, it is important to discern the source of that uncertainty. For instance, one source is probability (eg, a 30% chance of recovery), which inherently generates uncertainty. A second source is ambiguity, which arises from imprecision in probability (eg, 30%-70% chance of recovery), expert disagreement/conflicting opinion, or lack of scientific evidence/information. A third common source is complexity, in which multiple causal factors operate in a case, leading to uncertainty about outcome or prognosis. In some instances, identifying the source of uncertainty may aid in resolving it. For instance, if uncertainty derives from differences in expert opinions, informing families that two experts would disagree and describing their different perspectives and opinions helps to inform families. Moreover, when providers openly express the limits of knowledge and the various sources of uncertainty, the conversation itself engenders trust, transparency, and a greater ability for the family to understand the prognosis and allow them to engage in decision-making.