Resilio Sync (formerly BitTorrent Sync) by Resilio, Inc. is a proprietary peer-to-peer file synchronization tool available for Windows, Mac, Linux, Android, iOS, Windows Phone, Amazon Kindle Fire and BSD. It can sync files between devices on a local network, or between remote devices over the Internet via a modified version of the BitTorrent protocol.
Although not touted by the developers as an intended direct replacement nor competitor to cloud-based file synchronization services, it has attained much of its publicity in this potential role.  This is mainly due to the ability of Resilio Sync to address many of the concerns in existing services relating to file storage limits, privacy, cost, and performance.
Several private pre-alpha builds of "SyncApp" were subsequently made available to a limited group of alpha testers between January 2013 and April 2013. In mid-April 2013, the name "SyncApp" was dropped in favor of "BitTorrent Sync".
As of 6 May 2013, more than a petabyte of anonymous data had been synced between users, with over 70 terabytes synced daily. 
As of 16 July 2013, more than eight petabytes of data had been synced using the software. 
BitTorrent Sync migrated from "alpha" to "beta", released an Android app, and introduced versioning on 17 July 2013. 
On 5 November 2013, BitTorrent announced the release of BitTorrent Sync Beta API and version 1.2 of the client, along with the milestone, having over 1 million monthly active users synced over 30 petabytes of data to date. 
As of 26 August 2014, there have been more than 10 million user installs and more than 80 petabytes of data synced between users. 
On 3 March 2015, the product finally exited beta as a commercial product, with the inclusion of a paid Pro version. 
On 9 September 2015, with the release of Sync 2.2, in the free version, the 10 folder limit that had been introduced with 2.0, was removed. 
On 21 January 2016 the release of Sync 2.3 introduced the Encrypted Folder  plus the ability to run as a Windows Service, to use the SDcard on Android and for paid users Selective Sync support in Linux 
On 1 June 2016, product and team were spun out of BitTorrent Inc. as an independent company, Resilio Inc. which will continue development of the product under the name Resilio Sync.   Former Bittorrent CEO Eric Klinker became the head of the new company.  
Resilio Sync synchronizes files using BitTorrent.  The user's data is stored on the user's local device instead of in a "cloud", therefore requiring at least two user devices, or "nodes," to be online to synchronize files between them. Resilio Sync encrypts data with an Advanced Encryption Standard AES-128 key in counter mode which may either be randomly generated or set by the user. This key is derived from a "secret" which can be shared to other users to share data. Data is sent between devices directly unless the target device is unreachable (e.g. behind a firewall), in which case the data will first be relayed via an intermediary node. Many devices can be connected simultaneously and files shared between them in a mesh networking topology.
There is no limit on the amount of data that can be synced, other than the available free space on each device. 
Current builds of Resilio Sync are available for the following operating systems:
- Microsoft Windows (Windows 7 or later 32 bit and 64 bit)
- OS X (10.8 or later)
- Linux (web interface, also GUI available for Debian derived systems)