Temperature, Temperature, Temperature. Then humidity. And rainfall. Throw in a lighting cycle, the space needed and an aestevation period and you have quite the tough nut to crack on your hands.
I have heard that it can be done, however, for all intensive purposes, they were kept in outdoor enclosures.
There are exceptions to this: Ambystoma mexicanum, and a select few Ambystoma tigrinum from sites where they that remain totally neotenic.