The Fifty-move rule states that a game of chess is considered drawn after fifty consecutive full moves without a capture or a pawn move. If the last move of such series delivers a checkmate, this takes precedence over the 50 move rule. Inside a chess program, the halfmove clock takes care of enforcing fifty-move rule. If the halfmove clock becomes greater or equal than 100, and the side to move has at least one legal move, a draw score should be assigned to that node, with appropriate protocol handling and game state transitions, if the node is already the root and there is no mate in one.
9.3 The game is drawn, upon a correct claim by the player having the move, if  a) he writes his move on his scoresheet, and declares to the arbiter his intention to make this move which shall result in the last 50 moves having been made by each player without the movement of any pawn and without any capture, or b) the last 50 consecutive moves have been made by each player without the movement of any pawn and without any capture.
Since July 01, 2014
Since July 01, 2014 75 moves without capture and pawn move end the game even without a claim 
9.6 If one or both of the following occur(s) then the game is drawn: a) the same position has appeared, as in 9.2b, for at least five consecutive alternate moves by each player. b) any consecutive series of 75 moves have been completed by each player without the movement of any pawn and without any capture. If the last move resulted in checkmate, that shall take precedence.
At the beginning of the nineties, when it has been proven that some endgames can be won only in a larger number of moves, there has been an attempt to complicate the rule with a series of exceptions, all of which has been scraped later on.
- Endgame Tablebases | DTZ50 - Depth to Zeroing in the context of the Fifty-move Rule
- Graph History Interaction (GHI)
- Halfmove Clock
- Transposition Table
- John Roycroft (1984). A Proposed Revision of the ‘50-Move Rule’. ICCA Journal, Vol. 7, No. 3
- Bob Herschberg, Jaap van den Herik (1993). Back to fifty. ICCA Journal, Vol. 16, No. 1
- Galen Huntington, Guy Haworth (2015). Depth to Mate and the 50-Move Rule. ICGA Journal, Vol. 38, No. 2 » Endgame Tablebases 
- Horrible move! by Tony Hedlund, CCC, May 07, 1998 » HIARCS
- 50 move rule question by Uri Blass, CCC, April 20, 1999
- Repetitions/50 moves and TT by Sergei Markoff, CCC, September 13, 2011
- Texel recipe to fix TT draws scores by Marco Costalba, CCC, June 23, 2012 » Texel
- Draw by 50 move rule or mate by Harald Lüßen, CCC, October 13, 2012
- Half Move Clock Confusion by HumbleProgrammer, OpenChess Forum, January 10, 2013 » Halfmove Clock, Repetitions
- FIDE's new rules for chess by Mark Lefler, CCC, July 20, 2014
- The 75 move rule by Steven Edwards, CCC, March 11, 2015
- Hashtable and 50 move rule draws by Stan Arts, CCC, May 24, 2016 » Transposition Table
- The new chess rules (5-fold repetition and 75-move draw) by Lyudmil Antonov, FishCooking, November 29, 2016 » Stockfish, Repetitions
- Have engines updated for fide 2014 draw rules? by Norm Pollock, CCC, January 28, 2017
- Losing by draw by Hamfer, OpenChess Forum, March 17, 2017
- Fifty move counter and Null move by Tamás Kuzmics, CCC, August 09, 2017 » Halfmove Clock, Null Move Pruning
- Fifty-move rule from Wikipedia
- Fide Handbook - E.I.01A. Laws of Chess
- Endgame tablebases with the fifty-move rule by Galen Huntington
- Computerschach - Eine Wette, die ich gerne verloren habe by Horst Wandersleben (German)