Abstract: IT is of interest to consider, in this the year of Newton's tercentenary, the present position of acceleration in Newtonian mechanics. The Newtonian equations of motion of a particle in free space are unaltered in form, as is well known, on transformation to any other frame in uniform relative motion, but this is not true for transformations to relatively accelerated frames. Hence the notion of “inertial frames”—frames for which the Newtonian dynamics holds good—frames for the existence of which, according to Einstein and Infeld in their book, “The Evolution of Physics”, physics has been unable to account. The existence of inertial frames appears to indicate that any particle in the universe possesses an absolute acceleration.