  We discuss Kaluza-Klein (KK) decomposition in 5-dimensional (5D) field
theories with orbifold compactification. Kinetic terms localized at orbifold
fixed points, which are inevitably present in any realistic model, modify the
standard KK mass spectrum and interactions of KK modes. This, in turn, can
significantly affect phenomenology of the orbifold models. As an example, we
discuss gauge coupling unification in N=1 supersymmetric 5D orbifold SU(5)
model. We have found that uncertainties in the predictions of the model related
to modification of the KK masses are large and essentially uncontrollable.
