What act in Hamlet is it when and polonius are the library?

You are likely thinking of the scene where Hamlet and Polonius are in the library in Act II, Scene II of Shakespeare's *Hamlet*.

This is a pivotal scene where:

* Hamlet feigns madness and speaks in riddles, questioning Polonius about his daughter Ophelia.

* Polonius, convinced Hamlet is mad due to Ophelia's rejection, decides to spy on him further.

* Hamlet delivers the famous "To be or not to be" soliloquy.

Let me know if you have any other questions about Hamlet!