 A wealth of theoretical results establish that models and theories which reproduce the predictions of quantum theory must be in some sense “contextual”. A model or theory is noncontextual when the properties attributed to the system (e.g., the value of some dynamical variable that the system is found to have) are independent of the manner in which those properties are measured or observed (the context), beyond the actual observation itself (for instance, the other properties that are measured in conjunction with the measurement). In classical mechanics, the properties that systems are found to have and that are attributed to them as a consequence of measurement do not in principle depend on the measurement context, so classical mechanics is noncontextual. The first contextuality theorems for quantum theory go back to Bell (1966) and Kochen and Specker (1967). They demonstrate that if one were to consider quantum measurements as deterministically uncovering the values of arbitrary pre-existing dynamical variables of quantum systems, then such a model would have to be contextual (that is, measured values would have to depend upon the context of measurement). Thus, no noncontextual deterministic local hidden variable model can reproduce the observed statistics of quantum theory.