Microstructure of polycrystalline materials can be described as grain boundary (GB) networks; hence, GBs play major role in a wide-range of polycrystal properties. Control over GB kinetics provides effective means to tailor polycrystal properties through material processing. Understanding and prediction of GB kinetic behavior are always central problems in materials science. In this thesis, these are realized based on a mechanistic model. The mechanistic model describes a GB as intrinsic GB structure decorated by extrinsic defects. GB kinetics is described by the transition among all the stable/metastable intrinsic GB structures and the kinetics of GB defects. A major type of GB line defect - disconnection - is firstly studied. Disconnection structure can be determined according to bicrystallography. Many kinds of GB kinetic behaviors - including GB roughening, intrinsic GB mobility, driven GB migration, interaction between GBs and defects from grain interiors - are explained/predicted consistently based on the nucleation, propagation and reaction of disconnections in the GB. The disconnection model is verified by Monte Carlo simulations and atomistic simulations. Then, beyond disconnection, all types of GB line defects are studied by exploration of GB states. GB states are sampled by varying microscopic degrees of freedom for each particular macroscopic GB geometry. It is demonstrated that the multiplicity of metastable GB states is, in general, very large. Statistics of these metastable GB states is applied to predict finite-temperature equilibrium and nonequilibrium properties. A metastable structural unit model is proposed to predict the metastable GB structure and energy, allowing for accurate determination of GB energy vs. misorientation based on a very small number of atomistic simulations. The application of disconnection model and GB state transition model is exemplified by the study of GB sink efficiency to point defects under irradiation. It is found that the GB sink effect can be modeled by Robin or Neumann boundary condition for a continuum diffusion equation. Finally, nonequilibrium GB structure in polycrystalline thin films is studied. The presence of low-angle tilt GBs and their emergence at the film surface create grain rotation out of the film surface and, thus, roughness of the film. Geometric analysis and atomistic simulation indicate that such out-of-plane grain rotation is driven by reduction of GB energy.