Magnetotellurics (MT) is an electromagnetic geophysical method that is sensitive to variations in subsurface electrical resistivity. Measurements of natural electric and magnetic fields are done in the time domain, where instruments can record for a couple of hours up to mulitple months resulting in data sets on the order of gigabytes. The principles of findability, accessibility, interoperability, and reuse of digital assets (FAIR) requires standardized metadata. Unfortunately, the MT community has never had a metadata standard for time series data. In 2019, the Working Group for Magnetotelluric Data Handling and Software (https://www.iris.edu/hq/about_iris/governance/mt_soft) was assembled by the Incorporated Research Institutions for Seismology (IRIS) to develop a metadata standard for time series data. This product describes the metadata definitions. Metadata Hierarchy: Survey -> Station -> Run -> Channel The hierarchy and structure of the MT metadata logically follows how MT time series data are collected. The highest level is "survey" which contains metadata for data collected over a certain time interval in a given geographic region. This may include multiple principle investigators or multiple data collection episodes but should be confined to a specific project. Next, a "station" which contains metadata for a single location over a certain time interval. If the location changes during a run, then a new station should be created and subsequently a new run under the new station. If the sensors, cables, data logger, battery, etc. are replaced during a run but the station remains in the same location, then this can be recorded in the "run" metadata but does not require a new station entry. A "run" contains metadata for continuous data collected at a single sample rate. If channel parameters are changed between runs, this would require creating a new run. If the station is relocated then a new station should be created. If a run has channels that drop out, the start and end period will be the minimum time and maximum time for all channels recorded. Finally, a "channel" contains metadata for a single channel during a single run, where "electric", "magnetic", and "auxiliary" channels have some different metadata to uniquely describe the physical measurement.