Modelers typically create system models assuming some degree of certainty in what they are describing. However, there is a need to understand how much uncertainty there is in their projections of what the system will do and how well it can do this. We will discuss a new standard from OMG called Precise Semantics for Uncertainty Modeling (PSUM) that specifies concepts of uncertainty, accuracy, precision, and related concepts, and we will describe how to PSUM concepts in modeling our systems.