Long wavelength thermal fluctuations of lipid membranes are adequately described by the Helfrich elastic model. On the other hand, fluctuations of wavelength comparable with bilayer thickness exhibit significant deviations from the prediction of the elastic model and are typically assumed to be dominated by microscopic surface tension due to protrusion of lipid molecules into the solvent. In this talk, we present evidence that the short wavelength modes of a lipid membrane are dominated by fluctuations of the tilt of lipid molecules with respect to the membrane normal rather than the microscopic surface tension. We obtain an expression for spectral intensity of the thermal membrane fluctuations by appealing to the Hamm-Kozlov model that accounts for both membrane bending and lipid tilt contributions to the total membrane energy but neglects the contributions of the microscopic surface tension. The tilt and the bending fluctuations obtained from our coarse-grained molecular dynamics simulations of a dipalymitoylphosphatidylcholine (DPPC) lipid bilayer show good agreement with the theory. Furthermore, the obtained tilt and bending moduli are in close agreement with experimentally determined values. The magnitude of the microscopic protrusion tension estimated from our simulations is significantly smaller than that of the tilt modulus. These results indicate that the membrane fluctuations can be adequately described by a macroscopic elastic model down to scales of interlipid distance provided one accounts for the tilt fluctuations. Moreover, the influence of the tilt fluctuations remains significant for wavelengths several times larger than the bilayer thickness and therefore should be taken into account in analysis of MD simulations of membrane bending fluctuations. The dominance of the tilt energy at short wavelengths is also expected to play a significant role in interactions of lipid bilayers with inclusions, such as proteins, as well as in highly curved non-bilayer phases.