µTorrent (or uTorrent; commonly abbreviated as "µT" or "uT") is a freeware, closed source BitTorrent client owned by BitTorrent, Inc.
It is the most widely used BitTorrent client outside China, where Xunlei is more popular. It gets the "µ" in its name from the SI prefix "micro-", referring to the program's small memory footprint: the program was designed to use minimal computer resources while offering functionality comparable to larger BitTorrent clients such as Vuze or BitComet.
The program has received consistently good reviews for its feature set, performance, stability, and support for older hardware and versions of Windows. A report showed that µTorrent is the second most popular BitTorrent client (after the Chinese Xunlei).
It is available for Microsoft Windows and Mac OS X. A µTorrent Server is available for Linux. All versions are written in C++.
The program has been in active development since its first release in 2005. Although originally developed by Ludvig Strigeus, since December 7, 2006, the code is owned and maintained by BitTorrent, Inc. The code has also been employed by BitTorrent, Inc. as the basis for version 6.0 and above of the BitTorrent client, a re-branded version of µTorrent.
µTorrent is shipped as a single stand-alone compressed executable file, installed at first run. Recent versions have included the ability to install themselves on first run. Small executable size is achieved by avoiding the use of many libraries, notably the C++ standard library and stream facilities, and creating substitutes written specifically for the program. The executable is then compressed to roughly half of its compiled and linked size using UPX.
