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.