"A thorough analysis of the data obtained on depleted, natural, and enriched uranium lattices has been made. Consideration of the possible sources of discrepancies between theory and experiment has led to a suspicion of the calculated thermal neutron diffusion lengths. A series of diffusion length measurements in non-multiplying lattices of lead-cadmium alloy has been initiated. An analysis of some early exponential experiments on lattices proposed for a neutron production reactor has been carried out in order to determine whether experimental results on these more complicated structures are consistent with the analysis carried out for the "clean" lattices."