Microsoft 365 Substrate

The Microsoft 365 Substrate is Microsoft's internal storage and indexing layer that backs Microsoft 365 services — mailboxes, files, Teams chats, OneDrive content, calendar events, contacts, and more — all stored consistently and queryable through Microsoft Graph. The Substrate is what makes Microsoft Search work across services, what powers Microsoft 365 Copilot grounding ("find relevant content from any of the user's accessible Microsoft 365 data"), and what enables cross-workload features like delta queries, change notifications, and search. Mostly invisible to admins and users — you don't configure the Substrate; you configure the services it backs. The architectural foundation that lets Microsoft 365 feel like one product rather than a federation of separate ones.