Field reversed plasmas have been observed to spin up and develop a destructive rotatingm=2 instability. While the instability threshold has been reasonably well characterized, the cause of rotation remains obscure. Of the several mechanisms proposed, the following is treated; end shorting on open field lines spins up that portion of the plasma; viscous drag then spins up the plasma on closed field lines. Theoretical models are developed to describe these processes. The predicted behavior falls into three regimes. For low aspect ratio (≲2.8) the plasma is highly viscous, rotates as a rigid body, and spins up to instability in a few Alfve´n transit times. At intermediate aspect ratio (2.8 to 3.5) the plasma is weakly viscous; it reaches instability in the viscous transfer time which lengthens very rapidly for higher aspect ratio. At high aspect ratio (≳3.5 to 4) spin up is slower that the resistive decay of the plasma, and instability should not arise. Most recent experiments fall into the strongly viscous regime and correlate with its predicted characteristics. However, one series of experiments exhibited no apparent rotational instability, and also produced the large plasma radii characteristic of high aspect ratio. The present model in the high aspect ratio, weakly viscous regime is consistent with these observations.