Three dimensional eutectic alloys, Al-Cu, Al-Fe and Ag-Cu, are investigated using SEM and TEM to characterize the microstructures, and in-situ uniaxial tensile testing in the high resolution SEM stage of a STEM to study the detailed deformation and fracture mechanisms. It is found that if predictions of the strength and fracture behaviour of these alloys are to be reliable, the effect of both the interlamellar spacings and the many different types of eutectic inhomogeneities must be accounted for.