The dynamics of biological membranes can be characterized and measured
in different ways. In scattering experiments as well as computer
simulations, local density fluctuations can be probed on different
time and length scales, and theoretically interpreted within the
framework of generalized hydrodynamics. In this talk, I report on the
calculation of dynamic structure factors from large-scale molecular
dynamics simulations. In this context, the dynamic structure factor,
S(k,w), is the
space and time Fourier transform of the density-density correlation
function. The simulations have enabled a thorough test of theoretical
predictions, especially of interest in the hydrodynamic limit. The
simulations allow for calculation of dynamic structure factors with
considerably better resolution of wave vectors and angular frequencies
than from experimental data. Different ways to distinguish dynamic
processes in the membrane on separate time scales are discussed as
well as the determination of membrane material constants from theory.