The requirements for the archiving of ALMA observation data are challenging: Not only are the expected rates of bservation and monitor data extremely high (0.5 eraByte/day), there is also the need to archive metadata about projects, proposals, observations, scheduling blocks, etc. in a flexible way that allows for changes in the structure of these data over the years. The ALMA archive is divided conceptually into three parts: (1) The BulkStore for the very observation data, (2) the MonitorStore for monitor data collected by all instruments, and (3) the XMLStore for metadata about observation and monitor data. The entities in the three distinct stores are highly interrelated. We will give an overview over the architecture of the ALMA archive with a special focus on XML storage. XML (eXtended Markup Language) was chosen not only as format for communicating data in the ALMA computing infrastructure, but also for archiving data, since it provides the required flexibility needed by the ALMA archive: XML is designed to represent semistructured data, i.e. data whose structure is irregular, changing over time or even unknown. This makes it the format of choice for software that has to work over many years, when changes in the underlying data structures are unavoidable.
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