Tigran Vartanovich Petrosian the reigning Champion. Besides playing in 10 Chess Olympiads (losing only 1 game from 139, and winning 16 medals) Petrosian also participated in all (8) European team championships that were held in his life-time. He won 8 team and 4 individual gold medals, and lost none of the 52 games he played.
Petrosian is generally considered as the hardest player to beat in the history of chess.
Boris Vasilievich Spassky the challenger. After narrowly losing the 1966 World Championship match, Spassky won the 2nd Piatigorsky Cup. In 1967 he won Corus chess tournament in Beverwijk (before it was moved to Wijk aan Zee) and tournament in Sochi.
This time Spassky won to became 10th World champion. Spassky's flexibility of style was the key to his victory over Petrosian.
Impact on the future
Petrosian continued to play in Candidates matches basically until his death. He participated in a record 10 straight World Championship cycles (from 1952-54 cycle to 1979-81 cycle) either as a candidate or as a World champion. He is the only person to go through the Interzonal and Candidates stages undefeated (1961-1963 cycle).
Petrosian died at relativly early age (55) of stomach cancer on August 13th, 1984 in Moscow.
Something interesting: First three World champions (Steinitz, Lasker, Capablanca) died in New York, all four deceased Soviet World champions (Botvinnik, Smyslov, Tal, Petrosian) died in Moscow. From remaining three deceased World champions Alekhine died in Estoril, Euwe died in Amsterdam, and Fischer in Reykjavik.
Petrosian and Korchnoi (2nd in the Candidates matches) were seeded into the next Candidates. Bent Lar.sen (3rd in the Candidates) was seeded into the next Interzonal.