Some 40,000 Atalanta fans will be at the San Siro to see their team go up against Valencia, but why do they play their Champions League games more than 60 kilometres away from their home stadium?
The Italian side, from Bergamo, have played their European fixtures away from their Stadio Atleti Azzurri d'Italia since qualifying for the Europa League in 2017.
They played their 2017/18 Europa League home fixtures at the MAPEI Stadium, Sassuolo's home stadium, around 200km away in Reggio Emilia because the Atleti Azzurri d'Italia didn't meet UEFA' minimum requirements.
However, UEFA gave the Atleti Azzurri d'Italia the green light to host Champions League match this season, but Atalanta chose to play at the San Siro in Milan.
This was because their home ground is currently undergoing renovation works that has seen their capacity reduced to 16,000 temporarily.
So, in order to make the most of playing in Europe's premier club competition, Atalanta chose the San Siro to have bigger attendances.
Atalanta have played in front of 26,000 against Shakhtar Donetsk, 34,000 against Manchester City and 28,000 against Dinamo Zagreb so far in the competition.
Now, Valencia will visit to play in front of 40,000 on Wednesday night for Atalanta's first Champions League knockout tie in their history.