Although hypochondriasis is associated with the costly use of unnecessary medical resources, this mental health problem remains largely neglected. A lack of clear conceptual models and valid measures has impeded accurate assessment and hindered progress. The Multidimensional Inventory of Hypochondriacal Traits (MIHT) addresses these deficiencies with scales that correspond to a 4-factor model. The MIHT was built with construct validity as a guiding principle and began with an item pool that broadly assessed dimensions identified in the literature. The items were administered to large samples; factor analyses of the responses led to item pool revisions and scale refinements. Multiple studies validated the final MIHT scales and 4-factor model; these findings suggest that the MIHT will contribute to theory and research.