Attuned (Attuned Container Format) is an extensible storage format for time series data and associated metadata. Attuned was created to address the need for a true multi-modal time-series file format that can hold simultaneously multiple diverse time-series data streams. A key requirement for Attuned was to be able to store essentially any kind of time series data streams with few or no limitations on the number of channels, sampling rates, value formats, and amount or type of associated meta-data. A core use case for the format, is to store data from multiple sensors, which measure diverse hysiological data (e.g., EEG and other brain signals, heart rate, motion capture, eye tracking, video, audio, and human interface device inputs), but also any other kind of states that relates to such measurements (e.g., simultaneous application state and events). Attuned was designed with the additional requirements in mind that the file contents shall be streamable (e.g., at the time of recording or playback), the contained streams can be time-synchronized, the file format is efficient in terms of storage space, and that the specification should be simple and compact.