Environmental assessment practices have not been as successful as they should be. We believe that this is in part because environmental assessors lack a clear and useful set of principles—that is, a theory of practice. We propose a theory that derives 19 principles from 3 axioms: 1) assessments inform environmental management decisions, 2) assessments are science based, and 3) management decisions accommodate multiple goals and constraints. The 1st axiom leads to principles that change the focus from good assessments to good decisions. The 2nd focuses assessments on the scientific needs of the decision maker rather than making policy judgments or consensus building. The 3rd axiom leads to integration across disciplines, scales, and types of evidence. This theory of assessment practice implies the need for a new framework for environmental assessment that is more integrative than existing frameworks and more focused on making decisions that resolve environmental problems. We believe that this theory of environmental assessment can lead to clear assessment practices that compel beneficial and confident environmental management.