Melt compounding was used to prepare conventional composites of montmorillonite clay and polyethylene (PE) as well as nanocomposites of exfoliated montmorillonite platelets dispersed in a maleated polyethylene (PE-g-MAn) matrix. The extent of clay platelet exfolialion in the PE-g-MAn nanocomposites was confirmed by X-ray diffraction and resulted in a significant reduction of the degree of crystallinity and increased polymer crystallization rates. Studies of non-isothermal crystallization kinetics suggested that the exfoliated clay promotes heterogeneous nucleation and two-dimensional crystallite growth. PE/clay composites behaved in a similar manner as conventional macrocomposites, exhibiting modest increases in their theological properties and Young's modulus. Conversely, the nanoscale dimensions of the dispersed clay platelets in the nanocomposites led to significantly increased viscous and elastic properties and improved stiffness. This was attributed to the high surface area between the polymer matrix and the exfoliated clay, which resulted in enhanced phase adhesion.