We have already seen that Newtonian mechanics is not covariant under Lorentz transformations. For example, a constant acceleration a leads to a velocity \(v(t)=at> c\) for \(t > c/a\) . The challenge, then, is to find a formulation of a relativistic, Lorentz covariant mechanics, which in the limit of small velocities turns again into Newton’s mechanics. For this purpose we consider point particles in the four-dimensional spacetime. We then apply this to motion with constant acceleration, and consider in detail the well-known twin paradox.