We report the first molecular model for the transition states (critical nuclei) that determine the rate of micelle formation and breakdown. These processes, and homogeneous nucleation of macroscopic phase separation are handled in a unified way. It turns out that with micelle-forming amphiphiles several new features appear that do not occur with ordinary phase separation. We predict that the activation free-energy barrier for micelle formation is maximal but finite at the critical micelle concentration (CMC) and decreases with concentration. The barrier for micelle breakdown vanishes at the CMC and increases with concentration. Furthermore, our results indicate formation of transient large aggregates upon quenches far beyond the CMC.