Onsager discovered a deep underlying symmetry in the thermodynamic description of irreversible processes for which he was awarded the 1968 Nobel Prize in chemistry. After a brief sketch of irreversible thermodynamics and its history, this paper describes Onsager's path to his discovery. It is primarily based on an interview with Onsager a few months before his death, with some details from Onsager's Nobel address. When asked to rank his work, he placed this work at the top.