The 2017–18 Bundesliga was the 55th season of the Bundesliga, Germany's premier football competition. It began on 18 August 2017 and concluded on 12 May 2018. The fixtures were announced on 29 June 2017. Following an offline test phase in the previous season, the video assistant referee system was used for the first time in the Bundesliga on a trial basis following approval from IFAB. Bayern Munich were the defending champions and won their 27th Bundesliga title on 7 April with five games to spare, winning a sixth consecutive title for the first time in their history. 1. FC Köln and Hamburger SV were relegated at the end of the season, with the latter therefore losing their status as the only ever-presents in Bundesliga history. Summary One of the managerial changes before the start of the season was at Borussia Dortmund, who had finished third the previous season. After sacking Thomas Tuchel, they hired the Dutchman Peter Bosz in June 2017, after he had led Ajax to the 2017 UEFA Europa League Final. Dortmund were also the German club involved in the biggest transfer of the summer, selling young French forward Ousmane Dembélé to Barcelona for an initial €105 million. Meanwhile, reigning champions Bayern Munich prepared for the season by breaking the league's transfer record in their purchase of French midfielder Corentin Tolisso from Lyon for €41.5 million. On 28 September 2017, Bayern sacked manager Carlo Ancelotti amidst reports of player unrest, despite the club sitting in third place. He was replaced by Jupp Heynckes in his fourth spell at the club. On 28 October, a 2–0 win over RB Leipzig put Bayern on top of the table for the first time in the season. In December, Dortmund sacked Bosz with the team in seventh, and replaced him with the Austrian Peter Stöger who himself had recently been dismissed by winless bottom team 1. FC Köln. In the January 2018 transfer window, Dortmund lost the season's second-top scorer, Gabonese forward Pierre-Emerick Aubameyang, who moved to Arsenal for a €63 million fee. They replaced him by bringing in Chelsea's Michy Batshuayi on loan. From December to February, Bayern went on a 10-match winning streak that ended with a goalless draw with Hertha BSC at the Allianz Arena, and managed 13 unbeaten until a 1–2 loss at Leipzig on 18 March. Dortmund remained unbeaten for 12 games between December and 31 March,