The Doctrine of Signatures (DOS) is found throughout the world. Most scholars dismiss it as a “primitive” or “prescientific” idea. Despite its long history, the doctrine has had little critical review. A careful evaluation of signatures suggests four things. (1) There is no evidence that morphological plant signatures ever led to the discovery of medicinal properties. Considering DOS in this manner is unproductive and largely untestable. (2) Signatures are post hoc attributions rather than a priori clues to the utility of medicinal plants. (3) It is productive to redefine signatures to include organoleptic properties associated with therapeutic value. Plants with strong odors or bitter tastes, for example, commonly are found in pharmacopoeias. (4) DOS should be considered for what it primarily is—a way of disseminating information. DOS fundamentally is a mnemonic and, therefore, is exceedingly valuable in traditional cultures.