In this essay we take a critical look at the concept of “null curriculum”—what schools do not teach—and ask two questions: 1) Can the term “null curriculum” be defined adequately for curriculum theorizing?, and 2) Is this concept in any way useful for practice? Our consideration of the null curriculum centers on its possible uses in terms of theory, research, and practice. In each of these areas, particular attention is given to the problems involved in clearly defining this rather ambiguous concept. We note that conceptions of curriculum play the dominant role in how null curriculum is defined, and point out that a given null curriculum can be identified only in relation to what is valued as educationally significant. Although the notion of null curriculum cannot be defined in precise terms, we conclude that it does have worthwhile application in certain practical areas of curriculum development and evaluation.