  Environments with procedurally generated content serve as important
benchmarks for testing systematic generalization in deep reinforcement
learning. In this setting, each level is an algorithmically created environment
instance with a unique configuration of its factors of variation. Training on a
prespecified subset of levels allows for testing generalization to unseen
levels. What can be learned from a level depends on the current policy, yet
prior work defaults to uniform sampling of training levels independently of the
policy. We introduce Prioritized Level Replay (PLR), a general framework for
selectively sampling the next training level by prioritizing those with higher
estimated learning potential when revisited in the future. We show TD-errors
effectively estimate a level's future learning potential and, when used to
guide the sampling procedure, induce an emergent curriculum of increasingly
difficult levels. By adapting the sampling of training levels, PLR
significantly improves sample efficiency and generalization on Procgen
Benchmark--matching the previous state-of-the-art in test return--and readily
combines with other methods. Combined with the previous leading method, PLR
raises the state-of-the-art to over 76% improvement in test return relative to
standard RL baselines.
