In this paper a computational model called dynamic medial axis (DMA) is proposed to describe the internal evolution of planar shapes. To define the DMA, a symbolic representation called matching list is proposed to depict the topological structure of the medial axis. As shown in this paper with provable properties, the DMA exhibits an interesting dynamic skeleton structure for planar shapes. Finally an important application of the proposed DMA — computing the medial axis of multiply-connected planar shapes with curved boundaries — is presented.