The primary purpose of this paper is to show that, when relativity must be taken into account, there is no need to revise the ordinary definition of a cross section. Rather, a slight rearrangement of factors will obtain Lorentz invariant quantities. To demonstrate invariance is quite simple. Next, the reaction rate density can easily be written down just by bringing together, from a particle viewpoint, the various factors we know on physical grounds should be present. Again, by judicious grouping, the result can be expressed as a product of factors that are separately invariant. We go on to discuss the Lorentz transformation of an angular distribution (without explicitly using the Lorentz transformation equations), the relativistic normalization of wave functions, and finally, the connection of our simple approach with S-matrix theory.