We show that ferroelectric multilayers are not simple capacitors in series (CIS) and treating these as CIS may lead to misinterpretation of experimental results and to erroneous conclusions. Here, we present a theoretical model of ferroelectric bilayers using basic thermodynamics taking into account the appropriate electrical boundary conditions and electrostatic fields. The spontaneous polarization mismatch in ferroelectric/ferroelectric (FE/FE), FE/paraelectric (FE/PE), and FE/dielectric (FE/DE) bilayers results in a non-linear electrostatic coupling which produces significant deviations in the overall dielectric response if it is computed using the simple capacitor-in-series (CIS) model. Our results show that the CIS approach is a good approximation only for DE/DE multilayers and for FE heterostructures if the individual layers are electrostatically screened from each other.