Plate kinematics represents the first evidence of plate tectonic processes and the basics of plate dynamic models. The kinematics of plates is defined by Euler pole and angular velocity, and the main difficulty in describing plate motions is the choice of the reference frame. This problem is solved doing relative analysis, keeping one of the two plates fixed: then, plate displacements can be observed directly and computed by data coming from geology, geophysics and space geodesy. In contrast, there is not a direct way to measure motions relative to the mantle, and they need to be inferred indirectly, using results of relative plate kinematic models. Plate motions in the mantle-framework are useful to understand why plates move and what forces are involved, and also represents the initial conditions for plate dynamic numerical modeling. Tectonic evolution at the base of the oceans is generally obtained, modeling with relative plate motions and steady-state processes. A better way to understand lithosphere/mantle interactions, at mid-ocean ridges, corresponds to plate kinematic analyses with respect to the mantle, modeling time-dependent tectonic processes. Numerical solutions for viscosity flow beneath plates, that thicken with increasing age, provide useful relationships between mantle temperature and thickness of the oceanic lithosphere.