This paper examines the problem of theoretically predicting effective shear modulus of honeycomb core materials Included in the analysis are the effects resulting from boundary conditions which prevent warpage of the cells It is shown that these warpage constraints have little effect on shear modulus except when the ratio of core cell length to its lateral dimension becomes relatively small A test procedure using a rigid shear frame is suggested for measuring effective shear modulus, and some test results obtained by this piocedure are compared with the theoretical values