We explore the evolution of the internal gas kinematics of star-forming galaxies from the peak of cosmic star formation atz 2 to today. Measurements of galaxy rotation velocity V rot , which quantify ordered motions, and gas velocity dispersion s g , which quantify disordered motions, are adopted from the DEEP2 and SIGMA surveys. This sample covers a continuous baseline in redshift over < < z 0.1 2.5, spanning 10 Gyr. At low redshift, nearly all sufficiently massive star-forming galaxies are rotationally supported (By z=2, 50% and 70% of galaxies are rotationally supported at low ( ) stellar mass, respectively. For, the percentage drops below 35% for all masses. From z=2 to now, galaxies exhibit remarkably smooth kinematic evolution on average. All galaxies tend toward rotational support with time, and higher-mass systems reach it earlier. This is largely due to a mass-independent decline in s g by a factor of 3 since z=2. Over the same time period, V rot increases by a factor of 1.5 in low-mass systems but does not evolve at high mass. These trends in V rot and s g are at a fixed stellar mass and therefore should not be interpreted as evolutionary tracks for galaxy populations. When populations are linked in time via abundance matching, s g declines as before and V rot strongly increases with time for all galaxy populations, enhancing the evolution in s V g rot . These results indicate that = z 2 is a period of disk assembly, during which strong rotational support is only just beginning to emerge.