A novel three-dimensional flow birefringence technique has been developed to measure the real time, three-dimensional, optical anisotropy in flowing polymer solutions. Full tensor optical rheometry utilizes three polarization-modulated beams to probe all independent elements of the refractive index tensor and is, thus, used as a quantitative probe of time-dependent, three-dimensional average molecular orientation. We present results for shear flow of polystyrene in a 6% semidilute solution of tricresyl phosphate over a range of shear rates. Although the time-dependent behavior ofN2has not been widely reported, probably because of experimental difficulties, it can be a powerful means of testing rheological models. Experimental measurements of the transient behavior ofN2and −N2/N1on inception and cessation of simple shear flow are compared with the predictions of the Doi–Edwards and Giesekus models. The Giesekus model is in qualitative agreement with most of the time-dependent features observed forN2and −N2/N1. On-flow inception overshoots inN2and −N2/N1are observed. Further, on-flow cessation,N2relaxes more slowly thanN1and −N2/N1shows a strong nonexponential increase to a constant value of ∼0.9 at long times. The Doi–Edwards model is less successful in predicting transient results but agrees quite well with observed steady-state dependence ofN2with shear rate.