Hi! I'm Mammas Baller. I'm just another summoner on EUW.
I'm an Amumu main, so I'm really happy to see that he's now one of the best junglers in League ATM.
This is my very first guide on this website, so I'd appreciate any constructive feedback, and I'll keep adding new info when it's needed.
Also, this guide is mainly focused on Amumu as a jungler, but I added a support build as well because it's fun to play and actually pretty good once you get the hang of it.
Note: I'll add more chapters about Items, Summoner Spells, etc. once I know how to make them look decent

Pros:

Decently powerful all game, at least post 6
Fantastic in teamfights
Good ganks after Lv 3-4
Can be built full tank or AP bruiser
Good synergy with many different comps

Cons:

Weak early game
Slow first clear
Very prone to couter-jungling
Kinda weak if bullied early

These are the runes and masteries that I feel are the most powerful options on Amumu. I've already covered my reasons for these in the notes, so make sure to read those if you want an explanation for my runes/masteries.