Tough question, because this play brings up so many ideas. I would say that there are 2 major themes, one is a political theme which is the role of Socialism. At the time the play was written, socialism was being adopted by governments all over the world, and many writers and thinkers were flocking to socialist ideas and organizations.

Another is basically the idea of "what is life about"? The characters in the play debate the meaning of life pretty much throughout the play. The character Yank is always pondering his position in society and what life is "really all about". The play explores this issue all the time.