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In Evaluating Critical Thinking Stephen Norris and Robert Ennis say: "Although it is tempting to think that certain [unstated] assumptions are logically necessary for an argument or position, they are not. So do not ask for them." Numerous introductory logic text authors and various highly visible standardized tests (e.g., the LSAT and GRE) presume that Norris and Ennis are wrong; the presumption is that many arguments have (unstated) necessary assumptions and readers and test takers can reasonably be expected to identify them. This paper proposes and defends criteria for determining necessary assumptions of arguments. Both theoretical and empirical considerations are brought to bear.