A footwear construction typically produces different biomechanical, physiological, and/or perceptual responses for different groups of individuals. Consequently, a given insert or shoe can be beneficial for one group, but ineffective or even detrimental for another group. This observation represents a key challenge in the attempt to develop athletic footwear constructions that improve athletic performance, reduce the risk of movement-related injuries, and/or enhance comfort. A functional group is a collection of individuals with the same functional solutions when executing a locomotion task, and when reacting to a locomotion task intervention, such as footwear. It was speculated that tailoring footwear constructions to the functional solutions of these groups might help to develop footwear that promotes comfort, health, and/or performance. Before footwear constructions can be matched to functional groups, the groups have to be identified. To date, however, no systematic methodological approach exists to identify functional groups. Therefore, the main objective of this doctoral thesis was to develop and test methodological approaches that lead to the identification of functional groups in running. The first approach applied pattern recognition techniques to running kinematics in order to identify functional groups. The second approach combined footwear comfort with kinematic and muscle activity data collected during running to identify functional groups. The results showed that both tested approaches successfully identified functional groups. The main finding of this thesis is therefore that functional groups can be identified (1) by utilizing pattern recognition techniques, and (2) by isolating individuals who choose the same footwear as comfortable and excluding all individuals from this groups who do not share the same biomechanical solutions. Therefore, this PhD research provides a set of tools that can be utilized to identify functional groups in order to gain a better understanding about their specific functional solutions, biological characteristics, and footwear requirements.