  The relation between equal-time and light-front wave functions is studied
using models for which the four-dimensional solution of the Bethe-Salpeter wave
function can be obtained. The popular prescription of defining the longitudinal
momentum fraction using the instant-form free kinetic energy and third
component of momentum is found to be incorrect except in the non-relativistic
limit. The only presently known way to obtain light-front wave functions from
rest-frame, instant-form wave functions is to boost the latter wave functions
to the infinite momentum frame. Despite this fact, we prove a relation between
certain integrals of the equal-time and light-front wave functions.
