  We discuss in this paper two ways of defining the concept of "effective field
theory": effective field theory defined by low energy effectiveness and
effective field theory defined by 4D effectiveness out of higher dimensions. We
argue that these two views are actually equivalent, that effective field
theories at low energy can in fact be regarded as field theories of higher
dimensions confined on a 4D spcaetime. We examine this idea through comparing
two different regularization schemes: Momentum Cutoff and Dimensional
Regularization, and through analyzing how fields can be localized on branes.
