Modular Utility for STatistical kNowledge Gathering (MUSTANG) is a data quality assurance resource that scientists and network operators can use to assess the quality of seismic data archived at the Incorporated Research Institutions for Seismology (IRIS) Data Management Center. It is a system for automated calculation of metrics on seismic waveforms and distribution of these metrics via web services. Measurements span the entirety of the archive and are updated as needed when data or metadata change. MUSTANG precalculates and stores most metrics in a database, but a select few are dynamically generated when a web service request is made. Users can access metrics through the web service interfaces and also through easy‐to‐use client tools. This article describes the creation and construction of MUSTANG, as well as current and envisioned uses of its data quality measurements.