I. INTRODUCTION Max Planck first noted in 1899 1,2 the existence of a system of units based on the three fundamental constants, These constants are dimensionally independent in the sense that no combination is dimen- sionless and a length, a time, and a mass, may be constructed from them. Specifically, using ℏ ≡ h/ 2 π = 1 . 05 × 10 − $^{34}$Js in preference to h , the Planck scale is The energy associated with the Planck mass is E$_{P}$ = M$_{P}$c 2 = 1 . 2 × 10 $^{19}$GeV . 3 The Planck scale is prodigiously far removed from the human scale of about a meter. Indeed we humans are much closer in order of magnitude to the scale of the universe, 10 $^{26}$m , than to the Planck scale! Present high-energy particle experiments involve energies only of order 10 $^{3}$GeV , and even the highest energy cosmic rays detected to date, about 10 $^{12}$GeV , are far below the Planck energy. The presence of$_{ℏ}$ in the Planck units in Eq.(2) indicates that the Planck scale is associated with quantum effects, c indicates that it is associated with spacetime, and G indicates that it is associated with gravity. We therefore expect that the scale is characteristic of quantum spacetime or quantum gravity, which is the present conventional wisdom. 3 We wish to illustrate in this paper that the Planck scale is the boundary of validity of our present standard theories of gravity and quanta. Essentially we travel six roads to the boundary using arguments based on thought experiments. It is beyond our present scope to cross the boundary and discuss current efforts toward theories of quantum gravity and spacetime; however in section IX we will briefly mention a few such efforts, and we refer the reader to a number of useful references. Observational confirmation of Planck scale effects is highly problematic. 3,8–10 We clearly cannot expect to do accelerator experiments at the Planck energy in the foreseeable future, 2 G = 6 . 67 × 10 − $^{11}$Nm$^{2}$/kg$^{2}$(or m $^{3}$/kg s$^{2}$) (1) c = 3 . 00 × 10 $^{8}$m/s h = 6 . 60 × 10 − $^{34}$Js (or kg m $^{2}$/s ) l$_{P}$ = $^{√}$$_{ℏ}$G c 3 = 1 . 6 × 10 − $^{35}$m, T$_{P}$ = $^{√}$$_{ℏ}$G c 5 = 0 . 54 × 10 − $^{43}$, (2) M$_{P}$ = $^{√}$$_{ℏ}$c G = 2 . 2 × 10 − $^{8}$kg