Very little information survives about the career of M. Cocceius Nerva before he became Roman Emperor in A.D. 96. His importance by the end of Nero's reign is demonstrated by the rewards bestowed on him in 65 after the suppression of the Pisonian conspiracy; thereafter he became ordinary consul with Vespasian in 71 and with Domitian in 90. In this paper the attempt is made to explain by plausible hypothesis why Nerva was so highly regarded by both Vespasian and Domitian, and also how and why he succeeded Domitian in 96.