What is a person pledging when s/he converts to Judaism? How is it possible that even among Orthodox authorities, all of whom base their rulings on the same texts, debates rage about what makes a conversion valid? The reason, we’ll see, is that in ruling on conversion, rabbis are actually defining not only 'WHO is a Jew?' but 'WHAT is a Jew?'. We’ll look at several Orthodox responsa to see that even Orthodox authorities disagree about the fundamental question: what does it mean to be a Jew?