The development of improved hearing aid interfacing to the ear canal based on understanding the dynamics of the ear canal will be discussed. The National Institute of Deafness and other Communication Disorders (NIDCD) has supported this research to characterize the human ear canal and to develop a novel approach to improve the physical fit of hearing aids. This instructional course will summarize the findings on the active ear canal and results on improving the physical fitting of hearing aids. Visual images of laser scans of impressions of ear canals will be shared which quantitatively define the changing character of the canal with jaw position. Clinical results (both qualitative and quantitative) on the practical issues of feedback and occlusion effect control will be presented. Progress will be reported on the development of improved shell/earmould designs to better deal with feedback and occlusion. For those who provide hearing aids, practical ways will be emphasized to compensate for active ear canals.