rt.representation theory - When are all characteristic l representations liftable

Another sufficient condition is that if $G$ is solvable, then for every prime $l$, every absolutely irreducible characteristic $l$ representation can be lifted to the complex numbers. In fact, solvability is not really necessary; $l$-solvability suffices.
This is the Fong-Swan theorem.

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Added later: Since groups with order not divisible by $l$ are trivially $l$-solvable,
this sufficient condition includes, and is more general than the the condition stated by Pete L. Clark.