Microsoft Drive Optimizer (formerly Disk Defragmenter) is a utility in Microsoft Windows designed to increase data access speed by rearranging files stored on a disk to occupy contiguous storage locations, a technique called defragmentation.

Versions through Windows NT 3.51 did not have an application programming interface for moving data clusters on hard disks.

Microsoft included file system control (FSCTL) commands to move clusters in the Windows NT 4.0 kernel,[4] which worked for both NTFS and FAT partitions.

[12] In Windows Vista, Disk Defragmenter includes an option to automatically run at scheduled times using Task Scheduler and uses low CPU priority and the newly introduced low priority I/O algorithm so that it can continue to defrag using reduced resources (less CPU and disk read/write activity) when the computer is in use.

The user interface has been simplified, with the color graph, progress indicator, disk analysis and fragmentation information being removed entirely.

If the fragments of a file are over 64 MB in size, the file is not defragmented if using the GUI; Microsoft has stated that this is because there is no discernible performance benefit since the time seeking such large chunks of data is negligible compared to the time required to read them.

According to Scott Hanselman of Microsoft, Windows 7[verification needed] and later do defragment a solid-state disk (SSD) but in a completely different way.

There is less incentive for defragmentation of SSDs because file fragmentation has less performance impact on them and they handle a finite number of storage cycles before their lifespan expires.

In addition, Microsoft Drive Optimizer is also responsible for performing the TRIM command on SSDs.