We hereby present a study on lateral diffusion of lipids in Langmuir monolayers. We apply atomistic molecular dynamics simulations to a model system whose composition is consistent with protein-free lung surfactant. Our main focus is on the assessment of the validity of the free volume theory for lateral diffusion and on the interpretation of the cross-sectional area and activation energy parameters appearing in the theory. We find that the diffusion results can be fitted to the description given by the free volume theory, but the interpretation of its parameters is not straightforward. While the cross-sectional area appears to be related to the hard-core cross-sectional area of a lipid, its role in the lateral diffusion process is unclear. Also, the activation energy derived using the free volume theory is different from the activation energy found through Arrhenius analysis, and its physical interpretation remains elusive. Finally, we find that lipid diffusion does not occur via rapid single-particle "jumps". Instead, lipids move in a concerted manner as loosely defined transient clusters, as observed earlier for lipid bilayers.