# Folders in the OMERO model

OMERO offers both Projects that contain only Datasets and Datasets that contain only Images. Images may thus be organized within a two-level container hierarchy.

Reflecting the June 2016 release of the OME Data Model, OMERO 5.3’s object model adds a new kind of container, the Folder. In many respects they are rather like Datasets: for example, Folders have a description, they may be annotated and they may contain Images. However, they are different from Datasets in important respects.

Folders may contain:

- Images

- Regions of Interest (ROIs)

- other Folders

or a heterogeneous mix of the above.

For organizing data one may use Folder hierarchies of arbitrary depth. Just as an Image may be in multiple Datasets at once, the same Folder may be in multiple Folders at once. However, there is an acyclicity constraint: a Folder may not contain itself, even indirectly.

The OMERO graphical clients offer very limited support for Folders. At present Folders may be most useful for those working with their data via the OMERO Application Programming Interface and its gateways or with the OMERO.cli obj plugin. The measurement tool in OMERO.insight shows the Folders that ROIs are in. OMERO.web is yet to provide support for Folders.