Eta Carinae has provided more incisive clues to the evolution of the most massive stars than any other individual object. Its luminosity is comparable to that of a very massive O3 star, but it appears to have evolved beyond the O3 stage, to the limit where its surface is rather unstable. We think that it has begun to lose mass semi-catastrophically, in outbursts which may recur at intervals of perhaps several hundred years. The suspected binary nature of Eta Carinae introduces some conceptual uncertainty but is most likely not crucial.