Acoustic metamaterials have evolved from academic curiosity to a vibrant field with diverse applications. This review traces their development from early findings on mass density and bulk modulus frequency dispersions in locally resonant structures to the broader functionalities enabled by negative constitutive parameters. Recent advancements include compact phase manipulation structures, superabsorption, and actively controllable metamaterials, as well as new directions in acoustic wave transport in moving fluids, elastic, and mechanical metamaterials, graphene-inspired composites, and structures described by non-Hermitian Hamiltonians. Many novel structures have expanded beyond the original definition of metamaterials, offering wave manipulation functionalities beyond natural materials. Acoustic metamaterials manipulate waves governed by Newton's law, fluid continuity, and thermodynamic equations. The acoustic wave equation in a homogeneous medium is given by ∇²P - (ρ/κ)∂²P/∂t² = 0, where P is pressure, ρ is mass density, and κ is bulk modulus. In metamaterials, these parameters can take unusual values, leading to unique wave behaviors. The concept of metamaterials has broadened to include subwavelength structures with non-natural functionalities. The spring-mass model demonstrates dynamic effective mass, where the system's apparent inertia varies with frequency. Effective mass density can exhibit frequency dispersion, leading to unusual wave characteristics. Antiresonance conditions can also produce frequency-dispersive properties, enabling strong wave attenuation and reflection. Effective bulk modulus can be achieved through Helmholtz resonators, where the effective bulk modulus becomes frequency-dependent. The symmetry of resonances influences the effective mass and bulk modulus. Double negativity can be achieved by combining different resonant structures or by tuning eigenmodes. Membrane-type metamaterials, such as DMRs, can exhibit mass and bulk modulus dispersions, double negativity, and superresolution. The effective mass density and impedance of DMRs are related to their ability to manipulate acoustic waves. The normal displacement decomposition reveals the coupling of propagating and evanescent waves, with the effective mass density determining wave behavior. The effective bulk modulus of coupled membrane resonators can lead to double negativity, enabling unique wave propagation characteristics. Superresolution and focusing beyond the diffraction limit are achieved through subwavelength waveguides and resonant structures, allowing imaging with subdiffraction resolution. Acoustic superlenses and hyperlenses enable perfect imaging by amplifying evanescent waves. Transformation acoustics allows the design of materials with spatially varying index, enabling wave manipulation through coordinate transformations. Acoustic cloaking can guide waves around objects, isolating them acoustically. However, practical implementations face challenges due to extreme parameter values and frequency dispersion.Acoustic metamaterials have evolved from academic curiosity to a vibrant field with diverse applications. This review traces their development from early findings on mass density and bulk modulus frequency dispersions in locally resonant structures to the broader functionalities enabled by negative constitutive parameters. Recent advancements include compact phase manipulation structures, superabsorption, and actively controllable metamaterials, as well as new directions in acoustic wave transport in moving fluids, elastic, and mechanical metamaterials, graphene-inspired composites, and structures described by non-Hermitian Hamiltonians. Many novel structures have expanded beyond the original definition of metamaterials, offering wave manipulation functionalities beyond natural materials. Acoustic metamaterials manipulate waves governed by Newton's law, fluid continuity, and thermodynamic equations. The acoustic wave equation in a homogeneous medium is given by ∇²P - (ρ/κ)∂²P/∂t² = 0, where P is pressure, ρ is mass density, and κ is bulk modulus. In metamaterials, these parameters can take unusual values, leading to unique wave behaviors. The concept of metamaterials has broadened to include subwavelength structures with non-natural functionalities. The spring-mass model demonstrates dynamic effective mass, where the system's apparent inertia varies with frequency. Effective mass density can exhibit frequency dispersion, leading to unusual wave characteristics. Antiresonance conditions can also produce frequency-dispersive properties, enabling strong wave attenuation and reflection. Effective bulk modulus can be achieved through Helmholtz resonators, where the effective bulk modulus becomes frequency-dependent. The symmetry of resonances influences the effective mass and bulk modulus. Double negativity can be achieved by combining different resonant structures or by tuning eigenmodes. Membrane-type metamaterials, such as DMRs, can exhibit mass and bulk modulus dispersions, double negativity, and superresolution. The effective mass density and impedance of DMRs are related to their ability to manipulate acoustic waves. The normal displacement decomposition reveals the coupling of propagating and evanescent waves, with the effective mass density determining wave behavior. The effective bulk modulus of coupled membrane resonators can lead to double negativity, enabling unique wave propagation characteristics. Superresolution and focusing beyond the diffraction limit are achieved through subwavelength waveguides and resonant structures, allowing imaging with subdiffraction resolution. Acoustic superlenses and hyperlenses enable perfect imaging by amplifying evanescent waves. Transformation acoustics allows the design of materials with spatially varying index, enabling wave manipulation through coordinate transformations. Acoustic cloaking can guide waves around objects, isolating them acoustically. However, practical implementations face challenges due to extreme parameter values and frequency dispersion.