The objective of this study was to develop and test a base layer concept that aids in thermal comfort while running in cold weather. The base layer concept was developed through a participatory design scenario where attributes that are important to runners who run more than 30 miles per week were identified and applied through concepts generated in quasi-experimental controlled design sessions and tested through wear trials in actual use environments. The outcome of this research is Thermic, a further refined prototype, based on wear test results. This project followed a Participatory Design (PD) process where runners (a.k.a. users) were active participants in the creation and testing of the base layer. PD methods are grounded in the context of use and involve users as inside and active contributors throughout each step of design and development process .