Acoustic metamaterials have opened up unprecedented possibilities for wave manipulation, and can be utilized to realize many novel and fascinating physical phenomena, such as acoustic self-collimation, cloaking, asymmetric transmission, and negative refraction. In this review, we explore the fundamental physics of acoustic metamaterials and introduce several exciting developments, including the realization of unconventional effective parameters, acoustic metasurface, total sound absorption, high-resolution imaging, parity-time-symmetric materials, and topological acoustics. Acoustic metamatetials with negative effective parameters that are not observed in nature expand acoustic properties of natural materials. Acoustic metasurfaces can exhibit wavefront-shaping capabilities, with thickness being much smaller than the wavelength. The precisely designed matematerials provide the new possibility of steering waves on a subwavelength scale, which can be used for acoustic high-resolution imaging beyond the diffraction limit. The metamaterial absorbers can achieve total sound absorption at low frequencies and exhibit broadband absorption spectrum. Moreover, structure designs guided by the topological physics further broaden the whole field of acoustic metamaterials. Phononic crystals have become aflexible platform for studying new physics and exotic phenomenarelated to topological phases. Finally, we conclude the developments of acoustic metamaterials, discuss the technical challenges, and introduce potential applications in this emerging field.