The Extensible Storage Engine (ESE/ESENT), also known as JET Blue, is an Indexed Sequential Access Method (ISAM)data storage technology. Its purpose is to allow applications to store and retrieve data via indexed and sequential access.

ESE provides transacted data update and retrieval. A crash recovery mechanism is provided so that data consistency is maintained even in the event of a system crash. Transactions in ESE are highly concurrent making ESE suitable for server and client applications. ESE caches data intelligently to ensure high performance access to data. In addition, ESE is lightweight making it suitable for auxiliary applications.
The ESE Runtime (ESENT.DLL) has shipped in every Windows release since Windows NT 3.5, with native x64 version of the ESE runtime shipping with x64 versions as well (including IA64), and ARM. ESE is available on Windows, all flavors (server and client) and SKUs. It is installed by default and does not require any specific roles, features or components to be installed. It works out of the box for any Windows installation.