Bayer Leverkusen have excelled in European football in recent years and reached the final of the Champions League in 2002, losing to Real Madrid.
"They are good side and it will be a massive test," said Robbie. "It's another great experience, especially for the younger players, and I think this will be our hardest game in Europe to date.
"We've played well so far. People talked about how tough Besiktas was before we travelled, it was a massive game and that was probably or best display of the season. You get a taste of games like that and it leaves you wanting more.
"As a player you want to be involved in these matches. The competition has been good for us and we're still in the Carling Cup as well, so we've plenty of games at the moment and I think the lads are handling it well."
Robbie Keane is expecting the toughest test of this season's UEFA Cup campaign at Bayer Leverkusen tonight.We go into our third match of the group stages with a 100 per cent record in the competition - victories over Slavia Prague home and away in the first round, Besiktas and Club Brugge in the group stage.