Abstract Objectives Health outcomes instruments assess diverse health concepts. Although item-level concepts are considered fundamental elements, the field lacks structures for evaluating and organizing them for decision making. This article proposes a grammar using item stems, response options, and recall periods to systematically identify item-level concepts. The grammar uses “core concept,” “evaluative component,” and “recall period” as intuitive terms for communicating with stakeholders. Better characterization of concepts is necessary for classifying instrument content and linking it to treatment benefit. Methods Items in 2 generic and 21 disease-specific instruments were evaluated to develop and illustrate the use of the grammar. Concepts were assigned International Classification of Functioning, Disability and Health codes for exploring the value that the grammar and a classification system add to the understanding of content across instruments. Results The 23 instruments include many core concepts; emotional function is the only concept assessed in all instruments. Concepts in disease-specific instruments show obvious patterns; for example, arthritis instruments focus on physical function. The majority of instruments used the same response options across all items, with five-point scales being the most common. Most instruments used one recall period for all items. Shorter recall periods were used for conditions associated with “flares,” such as chronic obstructive pulmonary disease and “skin disease.” Every diagnosis, however, showed variation across instruments in the recall period used. Conclusions This analysis indicates the proposed grammar’s potential for discerning the conceptual content within and between health outcomes instruments and illustrates its value for improving communication between stakeholders and for making decisions related to treatment benefit.