Aspects of the subject matter described herein relate to adaptive retention for backup data. In aspects, space may be needed on a backup storage in order to store additional backup data. In response, an indication that space is needed provided to a retention manager. The retention manager attempts to prune data on the backup storage to free up space on the backup storage. Before deleting a version on the backup storage, the retention manager determines whether deletion conditions are met. The retention manager may apply one or more retention policies in its efforts to free space. The retention manager may manage and free the space such that versions of files already stored are preserved for the longest possible time.