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All past releases and their contents can be browsed at https://github.com/archivesspace/archivesspace/releases here.
Sprints:
Each public release is the accumulation of several internal releases that are coincident with the ArchivesSpace sprint cycle.
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Sprint reports are cumulatively listed at https://archivesspace.atlassian.net/wiki/display/ADC/2015+Sprint+Reports here.