Decently big because of the Amsterdam Internet Exchange I think. There are 2 pretty big players, Flowtraders and Optiver. From what I know getting in requires

Decently big because of the Amsterdam Internet Exchange I think. There are 2 pretty big players, Flowtraders and Optiver. From what I know getting in requires a decently impressive resume and passing some tough tests. I also expect it requires a masters or even PhD in a very mathematical field. That is the context where I saw them at the very least. I feel like I'd stand a decent chance at getting in if I could bring up the required enthusiasm in the interviews. But I do have a decently high opinion of myself.