Upcoming-wave is the basis of several seismic migration methods that use the principle of 1-way-time by propagating only from the event point to the receiver. The migration method that uses this upcoming wave theory is the finite difference migration and Fourier transformation migration. In this paper, we will elaborate how the theories of these two migrations are related and described from scalar wave equations to various forms of upcoming wave approximations, both differential equation form of space-time (x-t) and frequency - wave number (f-k) form, along with their transformations.