Relative motion between the forefoot and the rearfoot in the transverse plane has not been previously measured three-dimensionally. Past research has indicated a possible relationship between static foot structure and dynamic function. The purposes of this thesis were to quantify relative forefoot ab-adduction, arch height and arch length three- dimensionally and dynamically, and to relate these kinematics to foot type. Video motion analysis was conducted on cadaver legs and human subjects under quasi-static and dynamic conditions. In vitro results suggested that patterns of forefoot ab- adduction may be foot type dependent. Statistically significant correlations were established between relative arch deformation and normalized forefoot adduction, arch height flattening and medial arch elongation during quasi-static tibial rotation in vivo. Statistically significant differences were found between rigid and flexible foot types for range of forefoot abduction and maximal forefoot adduction during walking, running and side-shuffling. No kinematic differences were found during walking between a conventional hiking boot and a prototype designed to exhibit forefoot abduction, arch flattening and medial lengthening. These results suggest that foot type based on arch stiffness is an effective discriminator of kinematic foot function.