# Why is the actor-critic algorithm limited to using on-policy data?

Why is the actor-critic algorithm limited to using on-policy data? Or can we use the actor-critic algorithm with off-policy data?

It's because, in the actor-critic algorithm, the objective function is an expectation under the $$\tau$$ of the policy. If we want to use off-policy data, we have to resort to importance sampling relative to the other policy.