In computer simulations the wavevector-dependent dielectric constant ϵ(k) may be calculated from the equilibrium fluctuations of the spatial Fourier components of the instantaneous polarization of the sample. It is shown that this relationship depends on the boundary conditions/dipolar interactions used in the simulation, not only at infinite but also at finite wavelength, and explicit expressions are given for all simulation techniques currently used. By means of these fluctuation formulas it is now possible to calculate the entirek-dependent dielectric constant from simulations with arbitrary boundary conditions. The formulas also permit a general discussion of the relative merits of various simulation techniques as well as of some quantitative aspects of the Ewald sum in particular.