This chapter focuses on the dynamical description of motion involving four-momentum. Time is not a parameter provided by the universe, something external to one's frame of reference, rather it is local to each reference frame, and the most local time is the proper time. Four-velocity is the rate of change of the coordinate differentials on the worldline with respect to the proper time. In pre-relativistic mechanics, conservation of linear momentum, angular momentum, and energy follow directly from Newton's second law. The center of momentum framecenter of momentum frame is the inertial reference frame in which the spatial part of the total four-momentum is zero. Compton scattering is the inelastic scattering of a photon by a free particle. The effect gives experimental confirmation that the momentum of electromagnetic radiation is carried by particles called photons. The proper time is an invariant quantity and provides an absolute means to parameterize the worldline.