Liège–Bastogne–Liège is considered one of the most arduous one-day races in the world because of its length and succession of steep climbs.
Every edition, about a dozen climbs – ranging in length, gradient and difficulty – are addressed, offering opportunities to attack.

The British magazine Cycling Weekly stated: 
In purely physical terms, this is probably the toughest classic: the climbs are long, most of them are pretty steep as well, and they come up with depressing frequency in the final kilometres.

Four-times winner Moreno Argentin said:
Riders who win in Liège are what we call fondisti - men with a superior level of stamina. [
The climb of] La Redoute is like the Mur de Huy in that it has to be tackled at pace, from the front of the peloton.
The gradient is about 14 or 15 per cent, and it comes after 220 or 230 kilometers, so you don't have to be a genius to work out how tough it is.
I remember that we used to go up with a maximum of 39 x 21 - it's not quite as steep as the Mur de Huy.
A lot of riders mistakenly think you should attack on the hardest part, but in reality you hurt people on the slightly flatter section that comes after this.

Liège is a race of trial by elimination, where it's very unlikely that a breakaway can go clear and decide the race before the final 100 km [62 mi].
You need to be strong and at the same time clever and calculating — in this sense it's a complete test of a cyclist's ability.
Liège–Bastogne–Liège is considered one of the most arduous one-day races in the world because of its length and succession of steep climbs. Every edition, about a dozen climbs – ranging in length, gradient and difficulty – are addressed, offering opportunities to attack.
The British magazine Cycling Weekly stated: 
In purely physical terms, this is probably the toughest classic: the climbs are long, most of them are pretty steep as well, and they come up with depressing frequency in the final kilometres.
Four-times winner Moreno Argentin said:
Riders who win in Liège are what we call fondisti - men with a superior level of stamina. [The climb of] La Redoute is like the Mur de Huy in that it has to be tackled at pace, from the front of the peloton. The gradient is about 14 or 15 per cent, and it comes after 220 or 230 kilometers, so you don't have to be a genius to work out how tough it is. I remember that we used to go up with a maximum of 39 x 21 - it's not quite as steep as the Mur de Huy. A lot of riders mistakenly think you should attack on the hardest part, but in reality you hurt people on the slightly flatter section that comes after this.
Liège is a race of trial by elimination, where it's very unlikely that a breakaway can go clear and decide the race before the final 100 km [62 mi]. You need to be strong and at the same time clever and calculating — in this sense it's a complete test of a cyclist's ability.
Liège–Bastogne–Liège