Using high spatial resolution helioseismic data taken with the TON (Taiwan Oscillation Network) instruments, we have developed a method, called "acoustic imaging", to probe the structure and properties of solar active regions. The method of acoustic imaging, based on a time-distance relationship, coherently collects observed acoustic signals at the surface to reconstruct acoustic signals in the solar interior. We have successfully obtained acoustic images of several sunspot groups for various depths below the surface. Signatures of magnetic regions are clearly revealed, both in the sense of acoustic absorption and phase shift. At the surface, correspondence of these features between K-line images and reconstructed acoustic images is obvious. They extend down to about 30 - 40 Mm below the surface. However, a firm conclusion cannot be drawn until the issue of depth resolution of this method is better understood. Acoustic absorption images constructed with p-mode signals in different frequency ranges are presented. It is found that acoustic absorption in magnetic regions is stronger for 3- and 4-mHz than for 5- and 6-mHz. The dependence on the spherical harmonic degree l is very weak.